Hematite particles and magnetic recording medium using hematite particles as non-magnetic particles for a non-magnetic undercoat layer

ABSTRACT

The present invention relates to acicular hematite particles containing aluminum of 0.05 to 50% by weight, calculated as Al, having an average major axial diameter of not more than 0.3 μm and a pH value of the particles of not less than 8, and containing soluble sodium salts of not more than 300 ppm soluble sodium, calculated as Na, and soluble sulfates of not more than 150 ppm soluble sulfate, calculated as SO 4 , which have an excellent dispersibility in a binder resin, and a magnetic recording medium which uses the hematite particles as non-magnetic particles for a non-magnetic undercoat layer, which has small light transmittance, smooth surface, high strength and high durability, and which is capable of suppressing the deterioration in the magnetic properties caused by a corrosion of the magnetic particles containing as a main ingredient in the magnetic recording layer.

BACKGROUND OF THE INVENTION

The present invention relates to hematite particles and a magnetic recording medium using hematite particles as non-magnetic particles for a non-magnetic undercoat layer. More particularly, the present invention relates to high-density acicular hematite particles which have an excellent dispersibility in a binder resin, and a magnetic recording medium which uses the hematite particles as non-magnetic particles for a non-magnetic undercoat layer, which has small light transmittance, smooth surface, high strength and high durability, and which is capable of suppressing the deterioration in the magnetic properties caused by a corrosion of the magnetic particles containing as a main ingredient (hereinafter refer to as "magnetic iron-based metal particles") in the magnetic recording layer.

With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been increasingly and strongly required to have a higher performance, namely, a higher recording density, higher output characteristic, in particular, an improved frequency characteristic and a lower noise level.

Various attempts have been made at both enhancing the properties of magnetic particles and reducing the thickness of a magnetic recording layer in order to improve these properties of a magnetic recording medium.

The enhancement of the properties of magnetic particles will first be described.

The properties which magnetic particles are required to have in order to satisfy the above-described demands on a magnetic recording medium, are a high coercive force and a large saturation magnetization.

As magnetic particles suitable for high-output and high-density recording, acicular magnetic iron-based metal particles which are obtained by heat-treating acicular goethite particles or acicular hematite particles in a reducing gas are widely known.

Although acicular magnetic iron-based metal particles have a high coercive force and a large saturation magnetization, since the acicular magnetic iron-based metal particles used for a magnetic recording medium are very fine particles having a particle size of not more than 1 μm, particularly, 0.01 to 0.3 μm, such particles easily corrode, and the magnetic properties thereof are deteriorated, especially, the saturation magnetization and the coercive force are reduced.

Therefore, in order to maintain the characteristics of a magnetic recording medium which uses magnetic iron-based metal particles as the magnetic particles, over a long period, it is strongly demanded to suppress the corrosion of acicular magnetic iron-based metal particles as much as possible.

A reduction in the thickness of a magnetic recording layer will now be described. Video tapes have recently been required more and more to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the short-wave region have come to be used, and as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small.

With respect to short wavelength signals, a reduction in the thickness of a magnetic recording layer is also strongly demanded in order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium. This fact is described, for example, on page 312 of Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Sogo Gijutsu Center Co., Ltd. (1982), ". . . the conditions for high-density recording in a coated-layer type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force Hc and residual magnetization Br, . . . and the coating film has a smaller thickness . . . ".

Development of a thinner film for a magnetic recording layer has caused some problems.

Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Friction and Abrasion of Magnetic Tape and Head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Synthetic Technology" (1987), published by the Publishing Department of Technology Information Center, ". . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the substrate (back surface roughness) so largely as to be approximately proportional, . . . , since the magnetic layer is formed on the substrate, the more smooth the surface of the substrate is, the more uniform and larger head output is obtained and the more the S/N ratio is improved."

Secondly, there has been caused a problem in the strength of a non-magnetic substrate such as a base film with a tendency of the reduction in the thickness of the non-magnetic substrate in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, ". . . Higher recording density is a large problem assigned t the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize the size of a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a substrate . . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth travel in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded . . . ."

The end portion of a magnetic recording medium such as a magnetic tape, especially, a video tape is judged by detecting a portion of the magnetic recording medium at which the light transmittance is large by a video deck. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultrafine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. For reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the present video tapes.

However, addition of a large amount of non-magnetic particles such as carbon black impairs not only the enhancement of the recording density but also the development of a thinner recording layer. In order to reduce the magnetization depth from the surface of the magnetic tape and to produce a thinner magnetic recording layer, it is strongly demanded to reduce, as much as possible, the quantity of non-magnetic particles such as carbon black which are added to a magnetic recording layer.

It is, therefore, strongly demanded that the light transmittance of a magnetic recording layer should be small even if the carbon black or the like which is added to the magnetic recording layer is reduced to a small amount. From this point of view, improvements in the substrate are now in strong demand.

There is no end to a demand for a higher performance in recent magnetic recording media. Since the above-described reduction in the thickness of a magnetic recording layer and a non-magnetic substrate lowers the durability of the surface of the magnetic recording layer and the magnetic recording medium, an improvement of the durability of the surface of the magnetic recording layer and the magnetic recording medium is in strong demand.

This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-298679, ". . . With the recent development in magnetic recording, a high picture quality and a high sound quality have been required more and more in recording. The signal recording property is, therefore, improved. Especially, finer and higher-density ferromagnetic particles have come to be used. It is further required to make the surface of a magnetic tape smooth so as to reduce noise and raise the C/N . . . . However, the coefficient of friction between the magnetic layer and an apparatus during the travel of the magnetic tape increases, so that there is a tendency of the magnetic layer of the magnetic recording medium being damaged or exfoliated even in a short time. Especially, in a video tape, since the magnetic recording medium travels at a high speed in contact with the video head, the ferromagnetic particles are apt to be dropped from the magnetic layer, thereby causing clogging on the magnetic head. Therefore, an improvement in the running stability of the magnetic layer of a magnetic recording medium is expected . . . ."

Various efforts have been made to improve the base for a magnetic recording layer with a demand for a thinner magnetic recording layer and a thinner non-magnetic substrate. A magnetic recording medium having at least one undercoat layer (hereinunder referred to "non-magnetic undercoat layer") comprising a binder resin and non-magnetic iron-based metal particles such as hematite particles which are dispersed therein, on a non-magnetic substrate such as a base film has been proposed and put to practical use (Japanese Patent Publication (KOKOKU) No. 6-93297 (1994), Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338 (1987), 63-187418 (1988), 4-167225 (1992), 4-325915 (1992), 5-73882 (1993), 5-182177 (1993), 5-347017 (1993), 6-60362 (1994), 9-22524 (1997), etc.)

For example, Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) discloses a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer formed on the non-magnetic substrate and produced by dispersing inorganic particles in a binder resin; and a magnetic layer formed on the non-magnetic undercoat layer and produced by dispersing ferromagnetic particles in a binder resin while the non-magnetic undercoat layer is wet; wherein the magnetic layer has a thickness of not more than 1.0 μm in a dried state, the non-magnetic undercoat layer contains non-magnetic inorganic particles with surface layers coated with an inorganic oxide, the inorganic oxide coating the surfaces of the non-magnetic inorganic particles contained in the non-magnetic undercoat layer is at least one selected from the group consisting of Al₂ O₃, SiO₂ and ZrO₂, and the amount of the inorganic oxide coating the non-magnetic inorganic particles is 1 to 21 wt % in the case of Al₂ O₃, 0.04 to 20 wt % in the case of SiO₂, and 0.05 to 15 wt % in the case of ZrO₂, base on the total weigh of the magnetic inorganic particles.

Japanese Patent Application Laid-Open (KOKAI) No. 6-60362 (1994) discloses a non-magnetic undercoat layer for a magnetic recording medium formed on a non-magnetic substrate, comprising a coating film composition containing non-magnetic particles and a binder resin; wherein the non-magnetic particles are non-magnetic particles constituted by acicular α-Fe₂ O₃ particles coated with an Al compound, and the non-magnetic particles constituted by acicular α-Fe₂ O₃ particles coated with an Al compound have an average major axial diameter of 0.05 to 0.25 μm, an average minor axial diameter of 0.010 to 0.035 μm, a particle size distribution of not more than 1.40 in geometrical standard deviation, and an aspect ratio (major axial diameter/minor axial diameter) of 2 to 20.

However, the above-described non-magnetic particles are not particles which contain aluminum substantially uniformly from the central portions to the surfaces of the particles but particles which have an aluminum compound on the surfaces thereof.

A magnetic recording medium which has small light transmittance, high strength, smooth surface and higher durability, and which is capable of suppressing a corrosion of the acicular magnetic iron-based metal particles dispersed in the magnetic recording layer, with reduction of the thickness of not only the magnetic recording layer but also the non-magnetic substrate is now in the strongest demand, but no such magnetic recording medium which sufficiently satisfies these conditions have ever been obtained.

The above-described magnetic recording media composed of a non-magnetic substrate and a non-magnetic undercoat layer produced by dispersing non-magnetic particles in a binder resin and formed on a non-magnetic substrate, have a small light transmittance, a smooth surface and a high strength, but the durability thereof is inconveniently poor.

This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993), ". . . Although the problem of surface roughness is solved by providing a magnetic layer as an upper layer after forming a thick non-magnetic undercoat layer on the surface of a substrate, the problem of the abrasion of a head and the problem of durability are not solved and still remain. This is considered to be caused because a thermoset resin is ordinarily used as a binder of the undercoat layer so that the magnetic layer is brought into contact with a head or other members without any cushioning owing to the hardened undercoat layer, and a magnetic recording medium having such an undercoat layer has a considerably poor flexibility.

In addition, it has been pointed out that in the known magnetic recording media, the magnetic iron-based metal particles which are dispersed in the magnetic recording layer cause a corrosion after production, thereby greatly deteriorating the magnetic properties.

Also, the demand for the enhancement of the surface smoothness in a magnetic recording medium has become increasingly stronger, and the thinner magnetic recording layer, the smoother surface is strongly demanded.

As a result of studies undertaken by the present inventors so as to solve the above-described problems, it has been found that by using specific acicular hematite particles as the non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium, the obtained magnetic recording medium has small light transmittance, smooth surface, high strength and high durability, and it is capable of suppressing the deterioration in the magnetic properties by preventing a corrosion of the magnetic particles in a magnetic recording layer. The present invention has been achieved on the basis of this finding.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide acicular hematite particles suitable as non-magnetic particles for a non-magnetic undercoat layer of a magnetic recording medium which has small light transmittance, smooth surface, high strength and high durability, and which is capable of suppressing the deterioration in the magnetic properties caused by a corrosion of the magnetic particles, especially, magnetic iron-based metal particles, contained in the magnetic recording layer.

It is another object of the present invention to provide a magnetic recording medium which has small light transmittance, high strength, smoother surface, and higher durability, and which is capable of suppressing the deterioration in the magnetic properties caused by a corrosion of the magnetic particles, especially, magnetic iron-based metal particles, contained in a magnetic recording layer.

To accomplish the aims, in an aspect of the present invention, there are provided acicular hematite particles containing aluminum of 0.05 to 50% by weight, calculated as Al, having an average major axial diameter of not more than 0.3 μm and a pH value of the particles of not less than 8, and containing soluble sodium salts of not more than 300 ppm soluble sodium, calculated as Na, and soluble sulfates of not more than 150 ppm soluble sulfate, calculated as SO₄.

In a second aspect of the present invention, there is provided acicular hematite particles containing aluminum of 0.05 to 50% by weight, calculated as Al; having a coating comprising at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon, which is coated on the surfaces of said acicular hematite particles; having an average major axial diameter of not more than 0.3 μm and a pH value of the particles of not less than 8; and containing soluble sodium salts of not more than 300 ppm soluble sodium, calculated as Na, and soluble sulfates of not more than 150 ppm soluble sulfate, calculated as SO₄.

In a third aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer comprising a coating film composition comprising non-magnetic acicular hematite particles set forth in first or second aspect and a binder resin, which is formed on said non-magnetic substrate; and a magnetic recording layer comprising a coating film composition comprising magnetic particles containing iron as a main ingredient and a binder resin, which is formed on said non-magnetic undercoat layer.

In a fourth aspect of the present invention, there is provided magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer comprising a coating film composition comprising non-magnetic acicular hematite particles and a binder resin, which is formed on said non-magnetic substrate; and a magnetic recording layer comprising a coating film composition comprising magnetic particles containing iron as a main ingredient and a binder resin, which is formed on said non-magnetic undercoat layer,

wherein said non-magnetic acicular hematite particles contain 0.05 to 50 wt % of aluminum, calculated as Al, within the particle, have an average major axial diameter of not more than 0.3 μm, a pH value of said particles of not less than 8, and contain soluble sodium salts of not more than 300 ppm soluble sodium, calculated as Na and soluble sulfates of not more than 150 ppm soluble sulfate, calculated as SO₄ ; and said magnetic particles containing iron as a main ingredient comprising iron and aluminum of 0.05 to 10% by weight, calculated as Al.

In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer comprising a coating film composition comprising non-magnetic acicular hematite particles set forth in first or second aspect and a binder resin, which is formed on said non-magnetic substrate; and a magnetic recording layer comprising a coating film composition comprising binder resin and magnetic particles containing iron as a main ingredient comprising 50 to 99% by weight of iron, 0.05 to 10% by weight of aluminum, and at least one selected from the group consisting of Co, Ni, P, Si, B, Nd, La and Y, which is formed on said non-magnetic undercoat layer.

In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer comprising a coating film composition comprising non-magnetic acicular hematite particles set forth in first or second aspect and a binder resin, which is formed on said non-magnetic substrate; and a magnetic recording layer comprising a coating film composition comprising a binder resin and magnetic particles containing iron as a main ingredient comprising 50 to 99% by weight of iron, 0.05 to 10% by weight of aluminum, and at least one rare earth metal selected from the group consisting of Nd, La and Y, which is formed on said non-magnetic undercoat layer.

In a seventh aspect of the present invention, there is provided a magnetic recording medium comprising: a non-magnetic substrate; a non-magnetic undercoat layer comprising a coating film composition comprising non-magnetic acicular hematite particles set forth in second aspect and a binder resin, which is formed on said non-magnetic substrate; and a magnetic recording layer comprising a coating film composition comprising a binder resin and magnetic particles containing iron as a main ingredient comprising iron as a main ingredient comprising iron and aluminum of 0.05 to 10% by weight, calculated as Al, which is formed on said non-magnetic undercoat layer.

DETAILED DESCRIPTION OF THE INVENTION

The constitution of the present invention will be explained in more detail.

Acicular hematite particles (1) as non-magnetic particles according to the present invention will first be described.

The acicular hematite particles according to the present invention contain 0.05 to 50 wt % of aluminum (calculated as Al) approximately uniformly within the particles.

It is possible to obtain acicular hematite particles containing aluminum approximately uniformly within the particles by in case where acicular goethite particles are produced by passing an oxygen-containing gas such as air into a suspension containing the iron-containing precipitate such as iron hydroxide, iron carbonate or the like obtained by reacting a ferrous salt with an alkali hydroxide and/or an alkali carbonate, passing of the oxygen-containing gas into the suspension in the presence of the aluminum compound, thereby producing acicular goethite particles containing aluminum substantially uniformly from the central portions to the surfaces of the particles, and dehydrating the acicular goethite particles.

When the aluminum content within the acicular hematite particles is less than 0.05 wt % (calculated as Al), a magnetic recording medium having a non-magnetic undercoat layer containing such acicular hematite particles does not have a sufficient durability. If the aluminum content exceeds 50 wt %, although a magnetic recording medium having the non-magnetic undercoat layer containing such acicular hematite particles has a sufficient durability, the durability-improving effect becomes saturated, so that it is meaningless to add aluminum more than necessary. From the point of view of durability of a magnetic recording medium, the aluminum content therein is preferably 0.5 to 50 wt %, more preferably 1.0 to 50 wt %.

The acicular hematite particles in the present invention have an aspect ratio (average major axial diameter/average minor axial diameter) (hereinunder referred to merely as "aspect ratio") of not less than 2:1, preferably not less than 3:1. The upper limit of the aspect ratio is ordinarily 20:1, preferably 10:1 with the consideration of the dispersibility in the vehicle. The shape of the acicular particles here may have not only acicular but also spindle-shaped, rice ball-shaped or the like.

If the aspect ratio is less than 2:1, it is difficult to obtain a desired film strength of the magnetic recording medium.

The average major axial diameter of the acicular hematite particles of the present invention is not more than 0.3 μm, preferably 0.005 to 0.3 μm. If the average major axial diameter exceeds 0.3 μm, the particle size is so large as to impair the surface smoothness. On the other hand, if the average major axial diameter is less than 0.005 μm, dispersion in the vehicle may be unfavorably apt to be difficult. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coated film, the more preferable average major axial diameter is 0.02 to 0.2 μm.

The average minor axial diameter of the acicular hematite particles of the present invention is preferably 0.0025 to 0.15 μm. If the average minor axial diameter is less than 0.0025 μm, dispersion in the vehicle may be unfavorably difficult. On the other hand if the average minor axial diameter exceeds 0.15 μm, the particle size may be apt to become so large as to impair the surface smoothness. With the consideration of the dispersibility in the vehicle and the surface smoothness of the coated film, the more preferable average minor axial diameter is 0.01 to 0.10 μm.

The pH value of the acicular hematite particles of the present invention is not less than 8. If it is less than 8, the magnetic iron-based metal particles contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic properties. With the consideration of a corrosion preventive effect on the magnetic iron-based metal particles, the pH value of the particles is preferably not less than 8.5, more preferably not less than 9.0. The upper limit is ordinarily 12, preferably 11, more preferably 10.5.

The content of soluble sodium salts in the acicular hematite particles of the present invention is not more than 300 ppm soluble sodium (calculated as Na). If it exceeds 300 ppm, the magnetic iron-based metal particles contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic properties. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and efflorescence is sometimes caused in a highly humid environment. With the consideration of a corrosion preventive effect on the magnetic iron-based metal particles, the content of soluble sodium salt is preferably not more than 250 ppm, more preferably not more than 200 ppm, even more preferably not more than 150 ppm. From the point of view of industry such as productivity, the lower limit thereof is preferably about 0.01 ppm.

The content of soluble sulfate in the acicular hematite particles of the present invention is not more than 150 ppm soluble sulfate (calculated as SO₄). If it exceeds 150 ppm, the magnetic iron-based metal particles contained in the magnetic recording layer formed on the non-magnetic undercoat layer are gradually corroded, thereby causing a deterioration in the magnetic properties. In addition, the dispersion property of the acicular hematite particles in the vehicle is easily impaired, and the preservation of the magnetic recording medium is deteriorated and efflorescence is sometimes caused in a highly humid environment. With the consideration of a corrosion preventive effect on the magnetic iron-based metal particles, the content of soluble sulfate is preferably not more than 70 ppm, more preferably not more than 50 ppm. From the point of view of industry such as productivity, the lower limit thereof is preferably about 0.01 ppm.

The BET specific surface area of the acicular hematite particle of the present invention is ordinarily not less than 35 m² /g. If it is less than 35 m² /g, the acicular hematite particles may be coarse or sintering may be sometimes caused between particles, which are apt to exert a deleterious influence on the surface smoothness of the coated film. The BET surface area is more preferably not less than 40 m² /g, even more preferably not less than 45 m² /g, and the upper limit thereof is ordinarily 300 m² /g. The upper limit is preferably 100 m² /g, more preferably 80 m² /g with the consideration of the dispersibility in the vehicle.

The major axial diameter distribution of the acicular hematite particles of the present invention is preferably not more than 1.50 in geometrical standard deviation. If it exceeds 1.50, the coarse particles existent sometimes exert a deleterious influence on the surface smoothness of the coated film. The major axial diameter distribution is more preferably not more than 1.40, even more preferably not more than 1.35 in geometrical standard deviation with the consideration of the surface smoothness of the coated film. From the point of view of industrial productivity, the major axial diameter distribution of the acicular hematite particles obtained is ordinarily 1.01 in geometrical standard deviation.

In the acicular hematite particles of the present invention, the degree of densification is high. The degree of densification represented by the ratio of the specific surface area S_(BET) measured by a BET method and the surface area S_(TEM) calculated from the major axial diameter and the minor axial diameter which were measured from the particles in an electron micrograph is ordinarily 0.5 to 2.5.

With the consideration of the surface smoothness of the coated film and the dispersibility in the vehicle, the S_(BET) /S_(TEM) value is preferably 0.7 to 2.0, more preferably 0.8 to 1.6.

The resin adsorptivity of the acicular hematite particles of the present invention is ordinarily not less than 70%, preferably not less than 75%, more preferably not less than 80%.

The amount of sintering preventive existent on the surfaces of the acicular hematite particles of the present invention varies depending upon various conditions such as the kind of sintering preventive, the pH value thereof in an aqueous alkali solution and the heating temperature, it is ordinarily not more than 10 wt %, preferably 0.05 to 10 wt % based on the total weight of the particles.

The surfaces of the acicular hematite particles of the present invention may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon. When the acicular hematite particles coated with the above-described coating material are dispersed in a vehicle, they have an affinity with the binder resin and it is easy to obtain a desired dispersibility.

The amount of aluminum hydroxide, aluminum oxide, silicon hydroxide or silicon oxide used as the coating material is ordinarily not less than 50 wt %, preferably 0.01 to 50 wt % (calculated as Al or SiO₂). If it is less than 0.01 wt %, the dispersibility improving effect may be insufficient. If the amount exceeds 50.00 wt %, the coating effect becomes saturated, so that it is meaningless to add a coating material more than necessary. From the point of view of dispersibility in the vehicle, the preferable amount of coating material is preferably 0.05 to 20 wt % (calculated as Al or SiO₂).

Various properties of the acicular hematite particles coated with a coating material of the present invention, such as aspect ratio, average major axial diameter, average minor axial diameter, pH value, the content of soluble sodium salt, content of soluble sulfate, BET specific surface area, major axial diameter distribution, degree of densification, resin adsorptivity and amount of sintering preventive are approximately equivalent in values to those of the acicular hematite particles of the present invention the surfaces of which are not coated with a coating material.

The process for producing the acicular hematite particles according to the present invention will now be described.

In order to produce the acicular hematite particles of the present invention, acicular goethite particles containing aluminum within the particles are produced. Acicular goethite particles are produced by an ordinary method such as (A) a method of oxidizing a suspension having a pH value of not less than 11 and containing colloidal ferrous hydroxide particles which is obtained by adding not less than an equivalent of an alkali hydroxide solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto at a temperature of not higher than 80° C.; (B) a method of producing spindle-shaped goethite particles by oxidizing a suspension containing FeCO₃ which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution, by passing an oxygen-containing gas thereinto after aging the suspension, if necessary; (C) a method of growing acicular seed goethite particles by oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing acicular seed goethite particles, adding not less than an equivalent of an alkali hydroxide solution to the Fe²⁺ in the aqueous ferrous salt solution, to the aqueous ferrous salt solution containing the acicular goethite seed particles, and passing an oxygen-containing gas into the aqueous ferrous salt solution; and (D) a method of growing acicular seed goethite particles by oxidizing a ferrous hydroxide solution containing colloidal ferrous hydroxide particles which is obtained by adding less than an equivalent of an alkali hydroxide solution or an alkali carbonate solution to an aqueous ferrous salt solution, by passing an oxygen-containing gas thereinto, thereby producing acicular seed goethite particles, and growing the obtained acicular seed goethite particles in an acidic or neutral region.

The acicular goethite particles containing aluminum within the particles are obtained by passing the oxygen-containing gas such as air into the suspension of the iron-containing precipitate or aqueous solution of the ferrous salt, alkali hydroxide or alkali carbonate, which contain an aluminum compound.

It is essential that an aluminum compound is existent in the above-mentioned suspension or aqueous solution before passing the oxygen-containing gas such as air thereinto in the process for producing the goethite particles. To state this concretely, an aluminum compound may be added to any of the aqueous ferrous salt solution, the aqueous alkali hydroxide solution, the aqueous alkali carbonate solution and the suspension containing an iron-containing precipitate. It is the more preferable to add an aluminum compound to the aqueous ferrous salt solution.

Elements other than Fe and Al such as Ni, Zn, P and Si, which are generally added in order to enhance various properties of the particles such as the major axial diameter, the minor axial diameter and the aspect ratio, may be added during the reaction system for producing the goethite particles.

The acicular goethite particles obtained have an average major axial diameter of ordinarily 0.005 to 0.4 μm, an average minor axial diameter of ordinarily 0.0025 to 0.20 μm and a BET specific of about ordinarily 50 to 250 m² /g, and contain ordinarily soluble sodium salts of 300 to 1500 ppm soluble sodium (calculated as Na) and ordinarily soluble sulfates of 100 to 3000 ppm soluble sulfate (calculated as SO₄).

High-density acicular hematite particles containing aluminum within the particles are obtained by heating the acicular goethite particles containing aluminum within the particles at a temperature as high as not less than 550° C. In order to obtain high-density acicular hematite particles which retain the shapes of the acicular goethite particles, it is preferable to obtain low-density acicular hematite particles containing aluminum within the particles by heat-treating the acicular goethite particles at a low temperature, for example, 250 to 500° C. and then to heat the low-density acicular hematite particles at a high temperature, for example, not less than 550° C.

It is preferred to coat the particles with a sintering preventive before the heat-treatment at a low temperature or a high temperature in order to obtain high-density acicular hematite particles which retain the shapes of the acicular goethite particles. The acicular goethite particles coated with a sintering preventive contain soluble sodium salts of ordinarily 500 to 2000 ppm soluble sodium (calculated as Na) and soluble sulfates of ordinarily 300 to 3000 ppm soluble sulfate (calculated as SO₄), and have the BET specific surface area of ordinarily about 50 to 250 m² /g. The coating treatment using a sintering preventive is composed of the steps of: adding a sintering preventive to an aqueous suspension containing the acicular goethite particles, mixing and stirring the suspension, filtering out the particles, washing the particles with water, and drying the particles.

As the sintering preventive, sintering preventives generally used are usable. For example, phosphorus compounds such as sodium hexametaphosphate, polyphospholic acid and orthophosphoric acid, silicon compounds such as #3 water glass, sodium orthosilicate, sodium metasilicate and colloidal silica, boron compounds such as boric acid, aluminum compounds including aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride and aluminum borate, alkali aluminate such as sodium aluminate, and alumina sol, and titanium compounds such as titanyl sulfate may be exemplified.

The acicular hematite particles containing aluminum within the particles are obtained by heat-treating the acicular goethite particles containing aluminum within the particles at a temperature of 250 to 850° C.

The low-density acicular hematite particles obtained by heat-treating the acicular goethite particles coated with a sintering preventive at a temperature of 250 to 500° C. have an average major axial diameter of ordinarily 0.005 to 0.30 μm, an average minor axial diameter of ordinarily 0.0025 to 0.15 μm, a BET specific surface area of ordinarily about 70 to 350 m² /g and contain soluble sodium salts of ordinarily 500 to 2000 ppm soluble sodium (calculated as Na) and soluble sulfates of ordinarily 300 to 4000 ppm soluble sulfate (calculated as SO₄). If the temperature for heat-treating the goethite particles is less than 250° C., the dehydration reaction takes a long time. On the other hand, if the temperature exceeds 500° C., the dehydration reaction is abruptly brought out, so that it is difficult to retain the shapes because the sintering between particles is caused. The low-density acicular hematite particles obtained by heat-treating the goethite particles at a low temperature are low-density particles having a large number of dehydration pores through which H₂ O is removed from the goethite particles and the BET specific surface area thereof is about 1.2 to 2 times larger than that of the acicular hematite particles as the starting material.

The low-density hematite particles are then heat-treated at a temperature of not less than 550° C. to obtain a high-density acicular hematite particles. The upper limit of the heating temperature is preferably 850° C. The high-density hematite particles contain soluble sodium salts of ordinarily 500 to 4000 ppm soluble sodium (calculated as Na) and soluble sulfates of ordinarily 300 to 5000 ppm soluble sulfate (calculated as SO₄), and the BET specific surface area thereof is ordinarily about 35 to 150 m² /g.

If the heat-treating temperature is less than 550° C., since the densification is insufficient, a large number of dehydration pores exist within and on the surface of the hematite particles, so that the dispersion in the vehicle is insufficient. Further, when the non-magnetic undercoat layer is formed from these particles, it is difficult to obtain a coated film having a smooth surface. On the other hand, if the temperature exceeds 850° C., although the densification of the hematite particles is sufficient, since sintering is caused on and between particles, the particle size increases, so that it is difficult to obtain a coated film having a smooth surface.

The acicular hematite particles are pulverized by a dry-process, and formed into a slurry. The slurry is then pulverized by a wet-process so as to deagglomerate coarse particles. In the wet-pulverization, ball mill, sand grinder, colloid mill or the like is used until coarse particles having a particle size of at least 44 μm are substantially removed. That is, the wet-pulverization is carried out until the amount of the coarse particles having a particle size of not less than 44 μm becomes to not more than 10% by weight, preferably not more than 5% by weight, more preferably 0% by weight based on the total weight of the particles. If the amount of the coarse particles having a particle size of not less than 44 μm is more than 10% by weight, the effect of treating the particles in an aqueous alkali solution at the next step is not attained.

The acicular hematite particles with coarse particles removed therefrom are heat-treated in a slurry at a temperature of not less than 80° C. after the pH value of the slurry is adjusted to not less than 13 by adding an aqueous alkali solution such as sodium hydroxide.

The concentration of the alkali suspension containing the acicular hematite particles and having a pH value of not less than 13 is preferably 50 to 250 g/liter.

If the pH value of the alkali suspension containing the acicular hematite particles is less than 13, it is impossible to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the hematite particles, so that it is impossible to wash out the soluble sodium slat, soluble sulfate, etc. existing within and on the surfaces of the particles. The upper limit of the pH value is ordinarily about 14. If the effect of removing the solid crosslinking caused by the sintering preventive which exists on the surfaces of the hematite particles, the effect of washing out the soluble sodium slat, soluble sulfate, etc., and the effect of removing the alkali which adheres to the surfaces of hematite particles in the process of the heat-treatment of the aqueous alkali solution are taken into consideration, the preferable pH value is in the range of 13.1 to 13.8.

The heat-treating temperature in the aqueous alkali solution is preferably not less than 80° C., more preferably not less than 90° C. If the temperature is less than 80° C., it is difficult to effectively remove the solid crosslinking caused by the sintering preventive which exists on the surfaces of the hematite particles. The upper limit of the heating temperature is preferably 103° C., more preferably 100° C. If the heating temperature exceeds 103° C., although it is possible to effectively remove the solid crosslinking, since an autoclave or the like is necessary or solution boils under a normal pressure, it is not advantageous from the point of view of industry.

The acicular hematite particles heat-treated in the aqueous alkali solution are thereafter filtered out and washed with water by an ordinary method so as to remove the soluble sodium salt and soluble sulfate which are washed out of the interiors and the surfaces of the particles and the alkali adhered to the surfaces of the hematite particles in the process of heat-treatment with the aqueous alkali solution, and then dried.

As the method of washing the particles with water, a method generally industrially used such as a decantation method, a dilution method using a filter thickener and a method of passing water into a filter press is adopted.

If the soluble sodium salt and soluble sulfate which are contained within the hematite particles are washed out with water, even if soluble sodium salt and soluble sulfate adhere to the surfaces when the surfaces of the hematite particles are coated with a coating material in a subsequent step, for example, the later-described coating step, they can be easily removed by water washing.

The acicular hematite particles of the present invention are filtered out and washed with water by an ordinary method after they are heat-treated in the aqueous alkali solution. Thereafter, the acicular hematite particles may be coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon, if necessary.

In order to coat the hematite particles, an aluminum compound and/or a silicon compound is added to and mixed with an aqueous suspension under stirring which is obtained by dispersing the cake, slurry or dried particles of the acicular hematite particles into an aqueous solution. The pH value of the mixed solution is adjusted, if necessary. The acicular particles thus coated with at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon are then filtered out, washed with water, dried and pulverized. They may be further deaerated and compacted, if necessary.

As the aluminum compound for the coating, the same aluminum compounds as those described above as the sintering preventive are usable.

The amount of aluminum compound added is 0.01 to 50.00 wt % (calculated as Al) based on the weight of the acicular hematite particles. If the amount is less than 0.01 wt %, the improvement of the dispersibility in the vehicle may be insufficient. On the other hand, if the amount exceeds 50.00 wt %, the coating effect becomes saturated, so that it is meaningless to add an aluminum compound more than necessary.

As the silicon compound, the same silicon compounds as those described above as the sintering preventive are usable.

The amount of silicon compound added is 0.01 to 50.00 wt % (calculated as SiO₂) based on the weight of the acicular hematite particles. If the amount is less than 0.01 wt %, the improvement of the dispersibility in the vehicle may be insufficient. On the other hand, if the amount exceeds 50.00 wt %, the coating effect becomes saturated, so that it is meaningless to add an silicon compound more than necessary.

When both an aluminum compound and a silicon compound are used, the amount thereof used is preferably 0.01 to 50.00 wt % (calculated as Al and SiO₂) based on the weight of the acicular hematite particles.

A magnetic recording medium according to the present invention will now be explained.

The magnetic medium of according to the present invention comprises a non-magnetic substrate, a non-magnetic undercoat layer and a magnetic recording layer.

The non-magnetic undercoat layer in the present invention is produced by forming a coating film on the non-magnetic substrate and drying the coating film. The coating film is formed by applying a non-magnetic coating film composition which contains acicular hematite particles, a binder resin and a solvent, to the surface of the non-magnetic substrate.

As the non-magnetic substrate, the following materials which are at present generally used for the production of a magnetic recording medium are usable as a raw material: a synthetic resin such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonate, polyethylene naphthalate, polyamide, polyamideimide and polyimide; foil and plate of a metal such as aluminum and stainless steel; and various kinds of paper. The thickness of the non-magnetic substrate varies depending upon the material, but it is ordinarily about 1.0 to 300 μm, preferably 2.0 to 200 μm. In the case of a magnetic disc, polyethylene terephthalate is ordinarily used as the non-magnetic substrate. The thickness thereof is ordinarily 50 to 300 μm, preferably 60 to 200 μm. In the case of a magnetic tape, when polyethylene terephthalate is used as the non-magnetic substrate, the thickness thereof is ordinarily 3 to 100 μm, preferably 4 to 20 μm. When polyethylene naphthalate is used, the thickness thereof is ordinarily 3 to 50 μm, preferably 4 to 20 μm. When polyamide is used, the thickness thereof is ordinarily 2 to 10 μm, preferably 3 to 7 μm.

The thickness of the undercoat layer obtained by coating the non-magnetic substrate with a coating film composition and drying the coating film, is ordinarily 0.2 to 10.0 μm, preferably 0.5 to 5.0 μm. If the thickness is less than 0.2 μm, not only it is impossible to ameliorate the surface roughness of the base film but also the strength is insufficient.

As the binder resin in the present invention, the following resins which are at present generally used for the production of a magnetic recording medium are usable: vinyl chloride-vinyl acetate copolymer, urethane resin, vinyl chloride-vinyl acetate-maleic acid copolymer, urethane elastomer, butadiene-acrylonitrile copolymer, polyvinyl butyral, cellulose derivative such as nitrocellulose, polyester resin, synthetic rubber resin such as polybutadiene, epoxy resin, polyamide resin, polyisocyanate resin, electron radiation curing acryl urethane resin and mixtures thereof. Each of these resin binders may contain a functional group such as --OH, --COOH, --SO₃ M, --OPO₂ M₂ and --NH₂, wherein M represents H, Na or K. With the consideration of the dispersibility of the particles, a binder resin containing a functional group --COOH or --SO₃ M is preferable.

The mixing ratio of the acicular hematite particles with the binder resin is ordinarily 5 to 2000 parts by weight, preferably 100 to 1000 parts by weight based on 100 parts by weight of the binder resin.

It is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the non-magnetic undercoat layer.

The gloss of the coated film of the non-magnetic undercoat layer containing acicular hematite particles according to the present invention is ordinarily 180 to 280%, preferably 190 to 280%, more preferably 200 to 280% and the surface roughness Ra thereof is ordinarily 1.0 to 12.0 nm, preferably 1.0 to 10.0 nm, more preferably 2.0 to 9.0 nm, even more preferably 2.0 to 8.0 nm. The Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) thereof is ordinarily 125 to 150, preferably 127 to 150, more preferably 130 to 150.

The magnetic recording medium according to the present invention is produced by forming the non-magnetic undercoat layer formed on the non-magnetic substrate, forming a coating film on the non-magnetic undercoat layer by applying a coating film composition containing magnetic iron-based metal particles, a binder resin and a solvent, and drying the coating film to obtain a magnetic recording layer.

The magnetic particles containing iron as a main ingredient, that is, magnetic iron-based metal particles used in the present invention comprises iron and at least one selected from the group consisting of Co, Al, Ni, P, Si, Zn, Ti, Cu, B, Nd, La and Y. Further, the following magnetic iron-based metal particles may be exemplified.

1) Magnetic iron-based metal particles comprises iron; and Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles.

2) Magnetic iron-based metal particles comprises iron; and Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles.

3) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; and Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles.

4) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

5) Magnetic iron-based metal particles comprises iron; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

6) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

7) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

8) Magnetic iron-based metal particles comprises iron; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

9) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

10) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

11) Magnetic iron-based metal particles comprises iron; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

12) Magnetic iron-based metal particles comprises iron; Co of ordinarily 0.05 to 40 wt %, preferably 1.0 to 35 wt %, more preferably 3 to 30 wt % (calculated as Co) based on the weight of the magnetic iron-based metal particles; Al of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles; and at least one selected from the group consisting of Ni, P, Si/Zn, Ti, Cu and B of ordinarily 0.05 to 10 wt %, preferably 0.1 to 7 wt % (calculated as the corresponding element) based on the weight of the magnetic iron-based metal particles.

The iron content in the particles is the balance, and is preferably 50 to 99 wt %, more preferably 60 to 95 wt % (calculated as Fe) based on the weight of the magnetic iron-based metal particles.

The magnetic iron-based metal particles comprising (i) iron and Al; (ii) iron, Co and Al, (iii) iron, Al and at least one rare-earth metal such as Nd, La and Y, or (iv) iron, Co, Al and at least one rare-earth metal such as Nd, La and Y is preferable from the point of the durability of the magnetic recording medium. Further, the magnetic iron-based metal particles comprising iron, Al and at least one rare-earth metal such as Nd, La and Y is more preferable.

With respect to the existing position of aluminum of ordinarily 0.05 to 10 wt % (calculated as Al) based on the weight of the magnetic iron-based metal particles, it may be contained only in the core and inside portions, or in the surface portion of the magnetic iron-based metal particles. Alternatively, aluminum may be approximately uniformly contained in the magnetic iron-based metal particles from the core portion to the surface. An aluminum-coating layer may be formed on the surfaces of the particles. In addition, any of these positions may be combined. In the consideration of the effect of improving the surface property of the magnetic recording layer or the durability of the magnetic recording medium, magnetic iron-based metal particles uniformly containing aluminum from the core portion to the surface and coated with an aluminum-coating layer are more preferable.

When the content of aluminum is less than 0.05 wt % (calculated as Al), the adsorption of the resin to the magnetic iron-based metal particles in the vehicle may not be said sufficient, so that it may be difficult to produce a magnetic recording layer or a magnetic recording medium having a high durability. When the content of aluminum exceeds 10 wt %, the effect of improving the durability of the magnetic recording layer or the magnetic recording medium is observed, but the effect is saturated and it is meaningless to add aluminum more than necessary. Further more, the magnetic characteristics of the magnetic iron-based metal particles may be sometimes deteriorated due to an increase in the aluminum as a non-magnetic component. The existing amount of aluminum of the magnetic iron-based metal particles is preferably 0.1 to 7% by weight.

It is more preferable to produce a magnetic recording medium of the present invention using the magnetic iron-based metal particles containing Al and a rare-earth metal such as Nd, La and Y therein, because the magnetic recording layer or magnetic recording medium produced is apt to have a more excellent durability. Especially, magnetic iron-based metal particles containing Al and Nd therein are the even more preferable.

The acicular magnetic iron-based alloy particles containing aluminum within the particles are produced, as is well known, by adding an aluminum compound at an appropriate stage during the above-described process for producing acicular goethite particles to produce acicular goethite particles containing aluminum at desired positions of the particles, and heat-treating, at a temperature of 300 to 500° C., the goethite particles or the acicular hematite particles containing aluminum at desired positions within the particles which are obtained by dehydrating the goethite particles.

The acicular magnetic iron-based metal particles coated with aluminum are produced by heat-treating, at a temperature of 300 to 500° C., the acicular goethite particles coated with an oxide or hydroxide of aluminum, or the acicular hematite particles coated with the oxide-or hydroxide of aluminum which are obtained by dehydrating the goethite particles.

The acicular magnetic iron-based metal particles used in the present invention have an average major axial diameter of ordinarily 0.01 to 0.50 μm, preferably 0.01 to 0.30 μm, more preferably 0.01 to 0.20 μm, an average minor axial diameter of ordinarily 0.0007 to 0.17 μm, preferably 0.003 to 0.10 μm, and an aspect ratio of ordinarily not less than 3:1, preferably and not less than 5:1. The upper limit of the aspect ratio is ordinarily 15:1, preferably 10:1 with the consideration of the dispersibility in the vehicle. The shape of the acicular magnetic iron-based metal particles may have not only acicular but also a spindle-shaped, rice ball-shaped or the like.

As to the magnetic properties of the acicular magnetic iron-based metal particles used in the present invention, the coercive force is preferably 1200 to 3200 oe, more preferably 1500 to 2500 oe, and the saturation magnetization is ordinarily preferably 100 to 170 emu/g, more preferably 130 to 170 emu/g with the consideration of the properties such as high-density recording.

The resin adsorptivity of the acicular magnetic iron-based metal particles used in the present invention is ordinarily not less than 65%, preferably not less than 68%, more preferably not less than 70%, most preferably not less than 80%.

As the binder resin for the magnetic recording layer, the same binder resin as that used for the production of the non-magnetic undercoat layer is usable.

The thickness of the magnetic recording layer obtained by applying the film-coating composition to the non-magnetic undercoat layer and dried, is ordinarily in the range of 0.01 to 5.0 μm. If the thickness is less than 0.01 μm, uniform coating may be difficult, so that unfavorable phenomenon such as unevenness on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 μm, it may be difficult to obtain desired signal recording property due to an influence of diamagnetism. The preferable thickness is in the range of 0.05 to 1.0 μm.

The mixing ratio of the acicular magnetic iron-based metal particles with the binder resin in the magnetic recording layer is ordinarily 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight based on 100 parts by weight of the binder resin.

It is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the magnetic recording layer.

The magnetic recording medium according to the present invention has a coercive force of ordinarily 900 to 3500 oe, preferably 1000 to 3500 oe, more preferably 1500 to 3500 Oe; a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of ordinarily 0.85 to 0.95, preferably 0.86 to 0.95, more preferably 0.87 to 0.95; a gloss (of the coating film) of ordinarily 190 to 300%, preferably 200 to 300%, more preferably 210 to 300%; a surface roughness Ra (of the coating film) of ordinarily not more than 12.0 nm, preferably 1.0 to 11.0 nm, more preferably 2.0 to 10.0 nm, even more preferably 2.0 to 9.0 nm, most preferably 2.0 to 8.0 nm; a Young's modulus (relative value to a commercially available video tape: AV T-120 produced by Victor Company of Japan, Limited) of ordinarily not less than 130, preferably not less than 132; and a linear adsorption coefficient (of the coating film) of ordinarily 1.10 to 2.00 μm⁻¹, preferably 1.20 to 2.00 μm⁻¹. As to the durability, the running durability is ordinarily not less than 11 minutes, preferably not less than 15 minutes, more preferably not less than 20 minutes. In case using the magnetic iron-based metal particles containing Al as magnetic particles for the magnetic recording layer, the running durability is ordinarily not less than 18 minutes, preferably not less than 20 minutes, more preferably not less than 22 minutes. Also, the scratch resistance is ordinarily A or B, preferably A, when evaluated into four ranks: A, B, C and D.

The corrosiveness represented by a percentage (%) of change in the coercive force is ordinarily not more than 10.0%, preferably not more than 9.5%, more preferably not more than 8.0%, and the corrosiveness represented by a percentage (%) of change in the saturation magnetic flux density Bm is ordinarily not more than 10.0%, preferably not more than 9.5%, more preferably not more than 8.0%.

What is an important in the present invention is the fact that when the high-purity acicular hematite particles containing 0.05 to 50 wt % of aluminum (calculated as Al) within the particles, which have an average major axial diameter of not more than 0.3 μm, a pH value of not less than 8, which contain soluble sodium salts of not more than 300 ppm soluble sodium (calculated as Na) and soluble sulfates of not more than 150 ppm soluble sulfate (calculated as SO₄) are used as the non-magnetic particles for a non-magnetic undercoat layer, it is possible to enhance the strength of the substrate and the surface smoothness of the non-magnetic undercoat layer owing to the excellent dispersibility of the particles into a binder resin, and that when a magnetic recording medium is formed by using the non-magnetic undercoat layer, it is possible to-reduce the light transmittance, to enhance the strength and to make the surface of the magnetic recording layer more smooth.

The reason why the strength of the substrate is enhanced and the surface of the non-magnetic undercoat layer is made more smooth is considered to be as follows. Since the binder resin adsorptivity of the acicular hematite particles containing aluminum within the particles in the vehicle is enhanced, as will be shown in later-described examples, the degree of adhesion of the acicular hematite particles in the non-magnetic undercoat layer or the non-magnetic undercoat layer itself to the non-magnetic substrate is enhanced. Since it is possible to sufficiently remove the soluble sodium and the soluble sulfate, which agglomerate hematite particles by firmly crosslinking, by washing the particles with water, the agglomerates are separated into substantially discrete particles, so that acicular hematite particles having an excellent dispersion in the vehicle are obtained.

This fact will be explained in the following. The goethite particles as the starting material are produced by various methods, as described above. When the material for producing acicular goethite particles is ferrous sulfate in any method, a large amount of sulfate [SO₄ ⁻⁻ ] naturally exists in the goethite suspension.

Especially, when goethite particles are produced from an acidic solution, since water-soluble sulfate such as Na₂ SO₄ is simultaneously produced and an alkali metal such as K⁺, NH₄ ⁺ and Na⁺ are contained in the goethite suspension, a deposit containing an alkali metal and a sulfate is easily produced. This deposit is represented by RFe₃ (SO₄) (OH)₆ (R=K⁺, NH₄ ⁺, Na⁺). Such a deposit is a slightly soluble sulfuric acid-containing salt and cannot be removed by an ordinary water washing method. This slightly soluble salt becomes a soluble sodium salt or a soluble sulfate in the next heat-treatment step. The soluble sodium salt and soluble sulfate are firmly combined with the interiors or the surfaces of the acicular hematite particles by a sintering preventive, which is essential for preventing the deformation of the acicular hematite particles and sintering between particles in the heat-treatment at a high temperature for the densification of the particles and which is crosslinking the acicular hematite particles. In this manner, agglomeration between acicular hematite particles becomes further firmer. As a result, the soluble sulfate and the soluble sodium salt, especially, imprisoned in the interiors of the particles or the agglomerates become very difficult to remove by an ordinary water washing method.

When acicular goethite particles are produced in an aqueous alkali solution by using ferrous sulfate and sodium hydroxide, Na₂ SO₄ is simultaneously produced as a sulfate and NaOH exists in the mother liquor. Since they are both soluble, if the acicular goethite particles are adequately washed with water, Na₂ SO₄ and NaOH ought to be removed. However, since the crystallinity of acicular goethite particles is generally small, the water-washing effect is poor, and when the particles are washed with water by an ordinary method, the particles still contain water-soluble contents such as a soluble sulfate [SO₄ ⁻⁻ ] and a soluble sodium salt [Na⁺ ]. The water-soluble contents are firmly combined with the interiors or the surfaces of the acicular hematite particles by the sintering preventive which is crosslinking the particles, as described above, and the agglomeration between acicular hematite particles becomes further firmer. As a result, the soluble sulfate and the soluble sodium salt, especially, imprisoned in the interiors of the particles or the agglomerates become very difficult to remove by an ordinary water washing method.

It is considered that when the hematite particles in which the soluble sodium salt and the soluble sulfate are firmly combined with the interiors or the surfaces of the particles via the soluble sintering preventive, as described above, are pulverized by a wet-process so as to deagglomerate coarse particles, and heat-treated in the aqueous alkali solution having a pH value of not less than 13 at a temperature of not less than 80° C., the aqueous alkali solution sufficiently permeates into the interiors of the hematite particles, so that the binding strength of the sintering preventive which is firmly combined with the interiors and the surfaces of the particles, and the interiors of the agglomerates is gradually weakened, and the crosslinking is dissociated from the interiors and the surfaces of the particles and the interiors of the agglomerates, and simultaneously, the water-soluble sodium salt and the water-soluble sulfate are easily removed by water washing.

What is also important in the present invention is the fact that since the particles having a pH value of not less than 8, and containing soluble sodium salts of not more than 300 ppm soluble sodium (calculated as Na) and soluble sulfates of not more than 150 ppm soluble sulfate (calculated as SO₄) are used as the non-magnetic particles for the non-magnetic undercoat layer, it is possible to suppress the deterioration in the magnetic properties which is caused by the corrosion of the acicular magnetic iron-based metal particles dispersed in the magnetic recording layer.

It is considered that the deterioration in the magnetic properties which is caused by the corrosion of the acicular magnetic iron-based metal particles dispersed in the magnetic recording layer is suppressed because the contents of the soluble sodium salt and the soluble sulfate, which accelerate the corrosion of a metal, in the acicular hematite particles are small and the pH value of the hematite particles themselves is as high as not less than 8.

Actually, it is confirmed that a progress of corrosion of acicular magnetic iron-based metal particles was suppressed by a synergistic effect of a small soluble content and a pH value of not less than 8, from the fact that the advantages of the present invention was not attained in any of the cases of (i) heat-treating the hematite particles after wet-pulverization in a slurry with the pH value adjusted to less than 13 at a temperature of not less than 80° C., (ii) heat-treating the hematite particles in a slurry with the pH value adjusted to not less than 13 at a temperature of less than 80° C., and (iii) heat-treating the hematite particles containing coarse particles without being pulverized by a wet-process in a slurry with the pH value adjusted to not less than 13 at a temperature of not less than 80° C., as shown in later-described examples and comparative examples.

In addition, what is still another important in the present invention is that the surfaces of the magnetic recording layer and the magnetic recording medium of the present invention have an excellent durability.

The reason why the durability of the surfaces of the magnetic recording layer and the magnetic recording medium is enhanced is not clear yet, but it is considered that it is due to the effect of using the acicular hematite particles containing aluminum uniformly within the particles, having a pH value of not less than 8 and containing a small amount of soluble sodium salt and soluble sulfate as the non-magnetic particles. More specifically, it is considered that since the resin adsorptivity of the acicular hematite particles containing aluminum uniformly within the particles in the vehicles are enhanced due to the use of the above-described particles, as will be shown in later-described examples, the degree of adhesion of the acicular hematite particles in the non-magnetic undercoat layer or the non-magnetic undercoat layer itself to the non-magnetic substrate is enhanced.

When the acicular hematite particles of the present invention are used as the non-magnetic particles containing aluminum substantially uniformly within the particles for a non-magnetic undercoat layer, it is possible to produce a non-magnetic undercoat layer excellent in the strength of the substrate and the surface smoothness, and when a magnetic medium is produced by using the non-magnetic undercoat layer, it is possible to provide a magnetic medium having a small light transmittance, a smooth surface, a high strength and an excellent durability. That is, the acicular hematite particles are preferable as the non-magnetic particles for a non-magnetic undercoat layer of a high-density magnetic recording medium.

Especially, the magnetic medium of the present invention having a non-magnetic undercoat layer using the hematite particles of the present invention has a small light transmittance, a smooth surface, a high strength and an excellent durability and it is capable of suppressing the deterioration in the magnetic properties caused by a corrosion of the acicular magnetic iron-based metal particles in the magnetic recording layer. That is, the magnetic medium is preferable as a high-density magnetic medium.

Furthermore, due to the synergistic effect of using the acicular non-magnetic particles containing aluminum uniformly within the particles as the non-magnetic particles for the non-magnetic undercoat layer, and using the acicular magnetic iron-based metal particles containing aluminum as the magnetic particles for the magnetic recording layer, the durability is more excellent. That is, the magnetic medium is preferable as a high-density magnetic medium.

EXAMPLES

The present invention is described in more detail by Examples and Comparative Examples, but the Examples are only illustrative and, therefore, not intended to limit the scope of this invention.

Various properties of the lithium-iron oxide particles--produced according to the present invention were evaluated by the following methods.

(1) The residue on sieve after the wet-pulverization was obtained by measuring the concentration of the slurry after pulverization by a wet-process in advance, and determining the quantity of the solid content on the sieve remaining after the slurry equivalent to 100 g of the solid content was passed through the sieve of 325 meshes (mesh size: 44 μm).

(2) The average major axial diameter and the average minor axial diameter of the particles are expressed by the average values of 350 particles measured in the photograph obtained by magnifying an electron micrograph (×30000) by 4 times in the vertical and horizontal directions, respectively. The aspect ratio is the ratio of the average major axial diameter and the average minor axial diameter.

(3) The geometrical standard deviation (σg) of particle size distribution of the major axial diameter was obtained by the following method. The major axial diameters of the particles were measured from the magnified electron microphotograph in the above-mentioned (2). The actual major axial diameters of the particles and the number of particles were obtained from the calculation on the basis of the measured values. On logarithmico-normal probability paper, the major axial diameters were plotted at regular intervals on the abscissa-axis and the accumulative number of particles belonging to each interval of the major axial diameters was plotted by percentage on the ordinate-axis by a statistical technique. The major axial diameters corresponding to the number of particles of 50% and 84.13%, respectively, were read from the graph, and the geometrical standard deviation (σg) was measured from the following formula:

    Geometrical standard deviation (σg)={major axial diameter (μm) corresponding to 84.13% under integration sieve}/{major axial diameter (geometrical average diameter) corresponding to 50% under integration sieve}.

The smaller the geometrical standard deviation, the more excellent the particle size distribution of the major axial diameters of the particles.

(4) The specific surface area is expressed by the value measured by a BET method.

(5) The degree of denseness of the particles is represented by S_(BET) /S_(TEM) as described above. S_(BET) is a specific surface area measured by the above-described BET method. S_(TEM) is a value calculated from the average major axial diameter d cm and the average minor axial diameter w cm measured from the electron microphotograph described in (2) on the assumption that a particle is a rectangular parallellopiped in accordance with the following formula:

    S.sub.TEM (m.sup.2 /g)={(4·d·w.sup.- +2w.sup.2)/(d·w.sup.2 ·ρ.sub.p)}×10.sup.-4

wherein ρ_(p) is the true specific gravity of the hematite particles, and 5.2 g/cm³ was used.

Since S_(TEM) is a specific surface area of a particle having a smooth surface without any dehydration pore within or on the surface thereof, the closer S_(BET) /S_(TEM) of particles is to 1, it means, the smoother surface the particles have without any dehydration pore within or in the surface thereof, in other words, the particles are high-density particles.

(6) The content of each of Co, Al, Ti, P, Si, B and Nd was measured from fluorescent X-ray analysis.

(7) The pH value of the particles was measured in the following method. 5 g of the sample was weighed into a 300-ml triangle flask, and 100 ml of pure water was added. The suspension was heated and after keeping the boiled state for 5 minutes, it was corked and left to cool to an ordinary temperature. After adding pure water which was equivalent to the pure water lost by boiling, the flask was corked again, shaken for 1 minute, and left to stand for 5 minutes. The pH value of the supernatant obtained was measured in accordance JIS Z 8802-7.

(8) The contents of soluble sodium salts and soluble sulfates were measured by measuring the Na content and SO₄ ²⁻ content in the filtrate obtained by filtering the supernatant liquid produced for the measurement of pH value which is described above through filter paper No. 5C, by using an Inductively Coupled Plasma Emission Spectrophotometer (manufactured by Seiko Instruments and Electronics, Ltd.).

(9) The viscosity of the coating film composition was obtained by measuring the viscosity of the coating film composition at 25° C. at a shear rate D of 1.92 sec⁻¹ by using an E type viscometer EMD-R (manufactured by Tokyo Keiki, Co., Ltd.).

(10) The resin adsorotivitv of the particles represents the degree at which a resin is adsorbed to the particles. The closer to 100 the value obtained in the following manner, the better.

The resin adsorption Wa was first obtained. 20 g of particles and 56 g of a mixed solvent (27.0 g of methyl ethyl ketone, 16.2 g of toluene, and 10.8 g of cyclohexanone) with 2 g of a vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group dissolved therein were charged into a 100-ml polyethylene bottle together with 120 g of 3 mmφ steel beads. The particles and the solvent were mixed and dispersed by a paint shaker for 60 minutes.

Thereafter, 50 g of the coating film composition was taken out, and charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by the centrifugalization at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was determined by a gravimetric method and the resin content existing in the solid portion was determined by deducting the obtained resin content from the amount of the resin sharged as the resin adsorption Wa (mg/g) to the particles.

The total quantity of separated solid content was taken into a 100 ml-tall beaker, and 50 g of a mixed solvent (25.0 g of methyl ethyl ketone, 15.0 g of toluene, and 10.0 g of cyclohexanone) was added thereto. The obtained mixture was to ultrasonic dispersion for 15 minutes, and the thus-obtained suspension was charged into a 50-ml settling cylinder. The solid content was separated from the solvent portion by centrifuging them at a rate of 10000 rpm for 15 minutes. The concentration of the solid resin content contained in the solvent portion was measured so as to determine the resin content dissolved from the resin which had been adsorbed to the particle surfaces into the solvent phase.

The process from the step of taking the solid content into the 100 ml-tall beaker to the determination of the resin content dissolved into the solvent phase was repeated twice. The total quantity We (mg/g) of resin content dissolved into the solvent phase in the three cycles was obtained, and the value calculated in accordance with the following formula is expressed as the resin adsorptivity T(%):

    T(%)=[(Wa-We)/Wa]×100.

The higher T value, the firmer the resin adsorption to the particles surfaces in the vehicle and the more favorable.

(11) The gloss of the surface of the coating film of each of the non-magnetic undercoat layer and the magnetic recording layer was measured at an angle of incidence of 45° by a glossmeter UGV-5D (manufactured by Suga Shikenki, Co., Ltd.).

(12) The surface roughness Ra is expressed by the average value of the center-line average roughness of the profile curve of the surface of the coating film by using "Surfcom-575A" (manufactured by Tokyo Seimitsu Co., Ltd.).

(13) The durability of the magnetic medium was evaluated by the following running durability and the scratch resistance.

The running durability was evaluated by the actual operating time under the conditions that the load was 200 gw and the relative speed of the head and the tape was 16 m/s by using "Media Durability Tester MDT-3000" (manufactured by Steinberg Associates). The longer the actual operating time, the higher the running durability.

The scratch resistance was evaluated by observing through the microscope the surface of the magnetic tape after running and visually judging the degree of scratching. Evaluation was divided into the following four ranks.

A: No scratch

B: A few scratches

C: Many scratches

D: A great many scratches

(14) The strength of the coating film was expressed the Young's modulus obtained by "Autograph" (produced by Shimazu Seisakusho Ltd.). The Young's modulus was expressed by the ratio of the Young's modulus of the coating film to that of a commercially available video tape "AV T-120" (produce by Victor Company of Japan, Limited). The higher the relative value, the more favorable.

(15) The magnetic properties were measured under an external magnetic field of 10 kOe by "Vibration Sample Magnetometer VSM-3S-15 (manufactured by Toei Kogyo, Co., Ltd.)".

(16) The light transmittance is expressed by the linear adsorption coefficient measured by using "Photoelectric Spectrophotometer UV-2100" (manufactured by Shimazu Seisakusho, Ltd.). The linear adsorption coefficient is defined by the following formula, and the larger the value, the more difficult it is for the magnetic sheet to transmit light:

    Linear adsorption coefficient (μm.sup.-1)={1 n (1/t)}/FT

wherein represent a light transmittance (-) at λ=900 nm, and FT represents thickness (μm) of the coating film composition of the film used for the measurement.

(17) The thickness of each of the non-magnetic substrate, the non-magnetic undercoat layer and the magnetic recording layer constituting the magnetic recording medium was measured in the following manner by using a Digital Electronic Micrometer R351C (manufactured by Anritsu Corp.)

The thickness (A) of a non-magnetic substrate was first measured. Similarly, the thickness (B) (B=the sum of the thicknesses of the non-magnetic substrate and the non-magnetic undercoat layer) of a substrate obtained by forming a non-magnetic undercoat layer on the non-magnetic substrate was measured. Furthermore, the thickness (C) (C=the sum of the thicknesses of the non-magnetic substrate, the non-magnetic undercoat layer and the magnetic recording layer) of a magnetic recording medium obtained by forming a magnetic recording layer on the non-magnetic undercoat layer was measured. The thickness of the non-magnetic undercoat layer is expressed by B-A, and the thickness of the magnetic recording layer is expressed by C-B.

(18) The change in the magnetic properties with passage of time of a magnetic recording medium caused by the corrosion of the magnetic iron-based metal particles was examined as follows.

The magnetic recording medium was allowed to stand in an environment of a temperature of 60° C. and a relative humidity of 90% for 14 days, and the coercive force and the saturation magnetic flux density were measured before and after standing. A change in each characteristic was divided by the value before standing, and represented by percentage as a percentage of change.

(19) The light transmittance of a magnetic sheet is expressed by the linear adsorption coefficient measured by using "Photoelectric Spectrophotometer UV-2100" (manufactured by Shimazu Seisakusho, Ltd.), The linear adsorption coefficient is defined by the following formula:

    Linear adsorption coefficient (μm.sup.-1)={1 n (1/t)}/FT

wherein t represents light transmittance (-) at λ=900 nm, and FT represents thickness (μm) of the coating film composition of the film used for the measurement.

The larger the value, the more difficult it is for the magnetic sheet to transmit light.

As a blank for measuring the linear adsorption coefficient, the same non-magnetic base film as that of the above-mentioned magnetic sheet, was used.

Example 1

<Production of spindle-shaped hematite particles>

1200 g of spindle-shaped goethite particles containing 0.61 wt % of aluminum (calculated as Al) uniformly within the particles (average major axial diameter: 0.153 μm, average minor axial diameter: 0.0196 μm, aspect ratio: 7.80, BET specific surface area: 175.2 m² /g, content of soluble sodium salts: 1130 ppm soluble sodium (calculated as Na), content of soluble sulfates: 522 ppm soluble sulfate (calculated as SO₄), pH value of the particles: 7.9, geometrical standard deviation: 1.32), obtained from an aqueous ferrous sulfate solution, an aqueous aluminum sulfate solution and an aqueous sodium carbonate solution according to the method (B) was suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 9.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 30.0 g of #3 water glass as a sintering preventive, and after the end of addition, the obtained mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 5.8 by adding a 0.1-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with a silicon oxide and containing aluminum uniformly within the particles. The SiO₂ content in the spindle-shaped goethite particles was 0.72 wt %.

1000 g of the spindle-shaped goethite particles obtained were charged into a stainless steel rotary furnace, and heat-treated and dehydrated in the air at 300° C. for 60 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles containing aluminum uniformly within the particles. The thus-obtained low-density spindle-shaped hematite particles had an average major axial diameter of 0.115 μm, an average minor axial diameter of 0.0177 μm, an aspect ratio of 6.50, a BET specific surface area (S_(BET)) of 187.3 m² /g, and a S_(BET) /S_(TEM) value of densification of 4.00. The goethite particles contained soluble sodium salts of 1361 ppm soluble sodium (calculated as Na) and soluble sulfates of 568 ppm soluble sulfate (calculated as SO₄). The Al content was 0.67 wt %, the pH value of the particles was 7.8 and the geometrical standard deviation thereof was 1.34. The SiO₂ content in the low-density spindle-shaped hematite particles was 0.81 wt %.

650 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 640° C. for 20 minutes while rotating the furnace so as to conduct the sealing of dehydration pores. The thus-obtained high-density spindle-shaped hematite particles containing aluminum uniformly within the particles had an average major axial diameter of 0.110 μm, an average minor axial diameter of 0.0186 μm, an aspect ratio of 5.91, a BET specific surface area (S_(BET)) of 54.6 m² /g, and a S_(BET) /S_(TEM) value of densification of 1.22. The hematite particles contained soluble sodium salts of 3553 ppm soluble sodium (calculated as Na) and soluble sulfates of 3998 ppm soluble sulfate (calculated as SO₄). The pH value of the particles was 5.8 and the geometrical standard deviation was 1.36. The SiO₂ content in the hematite particles was 0.82 wt %. The resin adsorptivity thereof was 23.8%.

After 800 g of the high-density spindle-shaped hematite particles obtained were roughly pulverized by a Nara mill in advance, the obtained particles were charged into 4.7 l of pure water and peptized by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the high-density spindle-shaped hematite particles obtained was then mixed and dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured by S. C. Adichem, CO., Ltd.). The spindle-shaped hematite particles in the slurry remaining on a sieve of 325 meshes (mesh size: 44 μm) was 0% by weight.

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 100 g/liter, and 6N-aqueous NaOH solution was added to 7 liter of the slurry under stirring so as to adjust the pH value to 13.3. The slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 98 g/liter.

2 liter of the slurry washed with water was filtered through a Buchner filter, and pure water was passed until the electric conductivity of the filtrate became not more than 30 μs. The particles were then dried by an ordinary method and pulverized so as to obtain the target spindle-shaped hematite particles. The spindle-shaped hematite particles obtained contained 0.67 wt % of aluminum (calculated as Al) uniformly within the particles, and had an average major axial diameter of 0.110 μm, a minor axial diameter of 0.0185 μm, an aspect ratio of 5.95, a geometric standard deviation σg of particle size (major axial diameter) of 1.35, a BET specific surface (S_(BET)) of 54.0 m² /g, a S_(BET) /S_(TEM) value of densification of 1.20 and a pH value of the particles of 9.0. The spindle-shaped hematite particles contained soluble sodium salts of 138 ppm soluble sodium (calculated as Na) and soluble sulfates of 35 ppm soluble sulfate (calculated as SO₄). The resin adsorptivity thereof was 79.8%.

Example 2

<Production of a non-magnetic undercoat layer>

12 g of the spindle-shaped hematite particles containing 0.67 wt % of aluminum (calculated as Al) uniformly within the particles obtained in the Example 1 were mixed with a binder resin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 wt % of cyclohexanone) and cyclohexanone, and the mixture (solid content: 72 wt %) obtained was kneaded by a plasto-mill for 30 minutes.

The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binder resin solution (30 wt % of polyurethane resin having a sodium sulfonate group and 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the obtained mixture was mixed and dispersed by a paint shaker for 6 hours to obtain a coating film composition.

The thus-obtained coating film composition containing hematite particles was as follows:

    ______________________________________                                         Spindle-shaped hematite particles                                                                   100    parts by weight                                    Vinyl chloride-vinyl acetate                                                                        10     parts by weight                                    copolymer resin having a sodium                                                sulfonate group                                                                Polyurethane resin having a                                                                         10     parts by weight                                    sodium sulfonate group                                                         Cyclohexanone        44.6   parts by weight                                    Methylethyl ketone   111.4  parts by weight                                    Toluene              66.9   parts by weight                                    ______________________________________                                    

The viscosity of the obtained coating film composition was 384 cP. The coating film composition obtained containing hematite particles was applied to a polyethylene terephthalate film of 12 μm thick to a thickness of 55 μm by an applicator, and the film was then dried, thereby forming a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.5 μm.

The gloss of the coating film of the non-magnetic undercoat layer was 206%, the surface roughness Ra was 5.9 nm, and the Young's modulus (relative value) was 121.

<Production of a magnetic recording layer>

12 g of acicular magnetic iron-based metal particles (average major axial diameter: 0.11 μm, average minor axial diameter: 0.018 μm, aspect ratio: 6.10, coercive force: 1880 Oe, saturation magnetization: 128 emu/g), 1.2 g of a polishing agent (AKR-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.12 g of carbon black (#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 wt % of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78 wt %). The mixture was further kneaded by a plasto-mill for 30 minutes to obtain a kneaded material.

The thus-obtained kneaded material was charged into a 140 ml-glass bottle together with 95 g of 1.5 mmφ glass beads, a binder resin solution (30 wt % of polyurethane resin having a sodium sulfonate group and 70 wt % of a solvent (methyl ethyl ketone:toluene=1:1)), cyclohexanone, methyl ethyl ketone and toluene, and the mixture was mixed and dispersed by a paint shaker for 6 hours.

The thus-obtained magnetic coating film composition was as follows:

    ______________________________________                                         Iron-based alloy particles                                                                           100    parts by weight                                   Vinyl chloride-vinyl acetate                                                                         10     parts by weight                                   copolymer resin having a sodium                                                sulfonate group                                                                Polyurethane resin having a                                                                          10     parts by weight                                   sodium sulfonate group                                                         Polishing agent (AKP-30)                                                                             10     parts by weight                                   Carbon black (#3250B) 3.0    parts by weight                                   Lubricant (myristic acid: butyl                                                                      3.0    parts by weight                                   stearate 1:2)                                                                  Hardening agent (polyisocyanate)                                                                     5.0    parts by weight                                   Cyclohexanone         65.8   parts by weight                                   Methyl ethyl ketone   164.5  parts by weight                                   Toluene               98.7   parts by weight                                   ______________________________________                                    

The magnetic coating film composition obtained was applied to the non-magnetic undercoat layer to a thickness of 15 μm by an applicator, and the magnetic recording medium obtained was oriented and dried in a magnetic field, and then calendered. The magnetic recording medium was then subjected to a curing reaction at 60° C. for 24 hours, and thereafter slit into a width of 0.5 inch, thereby obtaining a magnetic tape. The thickness of the magnetic recording layer was 1.1 μm.

The magnetic tape obtained had a coercive force of 1960 Oe, a squareness (Br/Bm) of 0.87, a gloss of 235%, a surface roughness Ra of 6.0 nm, a Young's modulus (relative value) of 133, a linear absorption coefficient of 1.21, a running durability of 25.6 minutes, and a scratch resistance of A. Changes in the coercive force and the saturation magnetic flux density Bm with passage of time were 5.0%, and 4.2%, respectively.

Examples 3 to 17, Comparative Examples 1 to 15

<Types of acicular goethite particles>

The precursors 1 to 8 shown in Table 1 were used as the precursors for producing acicular hematite particles.

<Production of low-density acicular hematite particles>

Low-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of acicular goethite particles as the precursors, the kind and amount of sintering preventive, and heating and dehydration temperature and time. The particles obtained in Comparative Example 4 were goethite particles.

The main producing conditions and various properties are shown in Tables 2 to 5.

Examples 18 to 32, Comparative Examples 16 to 29

<Production of high-density acicular hematite particles>

High-density acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of low-density hematite particles, and the heating temperature and time for densification.

The main producing conditions and various properties are shown in Tables 6 and 7.

Examples 33 to 47, Comparative Examples 30 to 38

<Treatment of acicular hematite particles in an aqueous alkali solution>

Acicular hematite particles were obtained in the same way as in Example 1 except for varying the kind of high-density acicular hematite particles, whether or not the wet-pulverization process was conduced, whether or not the heat-treatment in the aqueous alkali solution was conducted, the pH value of the slurry, and the heating time and temperature.

The main producing conditions and various properties are shown in Tables 8 to 11.

Example 48

<Surface coating of spindle-shaped hematite particles>

The concentration of the slurry having a pH value 10.5 which was obtained in Example 33 by washing the particles in an aqueous alkali solution after heat-treatment with water by a decantation method was 98 g/liter. 5 liter of the slurry was re-heated to 60° C., and 907 ml (equivalent to 5.0 wt % (calculated as Al) based on the spindle-shaped hematite particles) of a 1.0-N NaAlO₂ solution was added to the slurry, and the mixture was held for 30 minutes. Thereafter, the pH value of the mixture was adjusted to 8.3 by using acetic acid. The particles were then filtered out, washed with water, dried and pulverized in the same way as in Example 1, thereby obtaining spindle-shaped hematite particles coated with a coating material.

The main producing conditions and various properties are shown in Tables 12 and 13.

Examples 49 to 62

Acicular hematite particles coated with a coating material were obtained in the same way as in Example 48 except for varying the kind of acicular hematite particles, and the kind and the amount of surface treating material.

The main producing conditions and various properties are shown in Table 12 and 13.

Examples 63 to 92, Comparative Examples 39 to 54

<Production of a non-magnetic undercoat layer>

A non-magnetic undercoat layer was obtained in the same way as in Example 2 by using the acicular hematite particles obtained in Examples 33 to 62, Comparative Examples 1, 3, 16 to 19, 24 and 30 to 38.

The main producing conditions and various properties are shown in Tables 14 to 16.

Examples 93 to 122, Comparative Examples 55 to 70

<Production of a magnetic recording medium using magnetic iron-based metal particles>

A magnetic recording medium using magnetic iron-based metal particles was obtained in the same way as in Example 2 except for varying the kind of non-magnetic undercoat layer obtained in Examples 63 to 92 and Comparative Examples 39 to 54 and the kind of acicular magnetic iron-based metal particles.

The main producing conditions and various properties are shown in Tables 17 to 19.

Example 123

<Production of spindle-shaped hematite particles>

1200 g of spindle-shaped goethite particles containing 0.83 wt % of aluminum (calculated as Al) uniformly within the particles (average major axial diameter: 0.178 μm, average minor axial diameter: 0.0225 μm, aspect ratio: 7.91, BET specific surface area: 160.3 m² /g, soluble sodium salts: 1232 ppm soluble sodium.(calculated as Na), soluble sulfates: 621 ppm soluble sulfate (calculated as SO₄), pH value of the particles: 6.7, geometrical standard deviation: 1.33) obtained from an aqueous ferrous sulfate solution, an aqueous aluminum sulfate solution and an aqueous sodium carbonate solution by the method (B) was suspended in water so as to obtain a slurry, and the concentration of the solid content was adjusted to 8 g/liter. 150 liter of the slurry was heated to 60° C. and the pH value of the slurry was adjusted to 9.0 by adding a 0.1-N aqueous NaOH solution.

To the alkali slurry was gradually added 36.0 g of #3 water glass as a sintering preventive, and after the end of addition, the mixture was aged for 60 minutes. The pH value of the slurry was then adjusted to 6.0 by adding a 0.5-N acetic acid solution. Thereafter, the particles were filtered out, washed with water, dried and pulverized by an ordinary method, thereby producing spindle-shaped goethite particles coated with a silicon oxide and containing aluminum uniformly within the particles. The SiO₂ content in the spindle-shaped goethite particles was 0.86 wt %.

1000 g of the spindle-shaped goethite particles obtained were charged into a stainless steel rotary furnace, and heat-treated and dehydrated in the air at 350° C. for 30 minutes while rotating the furnace, to obtain low-density spindle-shaped hematite particles. The thus-obtained low-density spindle-shaped hematite particles had an average major axial diameter of 0.134 μm, an average minor axial diameter of 0.0194 μm, an aspect ratio of 6.91, a BET specific surface area (S_(BET)) of 168.3 m² /g, and a S_(BET) /S_(TEM) value of densification of 3.96. The goethite particles contained soluble sodium salts of 1123 ppm soluble sodium (calculated as Na) and soluble sulfates of 465 ppm soluble sulfate (calculated as SO₄). The Al content was 0.91 wt %, the pH value of the particles was 6.3 and the geometrical standard deviation was 1.34. The SiO₂ content in the low-density spindle-shaped hematite particles was 0.94 wt %.

900 g of the low-density spindle-shaped hematite particles were then charged into a ceramic rotary furnace, and heat-treated in the air at 630° C. for 30 minutes while rotating the furnace so as to conduct the sealing of dehydration pores. The thus-obtained high-density spindle-shaped hematite particles containing aluminum uniformly within the particles had an average major axial diameter of 0.129 μm, an average minor axial diameter of 0.0206 μm, an aspect ratio of 6.26, a BET specific surface area (S_(BET)) of 46.6 m² /g, and a S_(BET) /S_(TEM) value of densification of 1.16. The hematite particles contained soluble sodium salts of 2864 ppm soluble sodium (calculated as Na) and soluble sulfates of 2956 ppm soluble sulfate (calculated as SO₄). The pH value of the particles was 5.4 and the geometrical standard deviation was 1.36. The SiO₂ content in the hematite particles was 0.94 wt %. The resin adsorptivity thereof was 21.6%.

After 800 g of the high-density spindle-shaped hematite particles obtained were roughly pulverized by a Nara mill in advance, the obtained spindle-shaped hematite particles were charged into 4.7 liter of pure water and peptized by a homomixer (manufactured by Tokushu-kika Kogyo, CO., Ltd.) for 60 minutes.

The slurry of the high-density spindle-shaped hematite particles obtained was then mixed and dispersed for 3 hours at an axial rotation frequency of 2000 rpm while being circulated by a horizontal SGM (Dispermat SL, manufactured d by S.C. Adichem, CO., Ltd.). The spindle-shaped hematite particles in the slurry remaining on a sieve of 325 meshes (mesh size: 44 μm) was 0% by weight.

The concentration of the high-density spindle-shaped hematite particles in the slurry was adjusted to 100 g/liter, and a 6N-aqueous NaOH solution was added to 7 liter of the slurry under stirring so as to adjust the pH value to 13.3. The slurry was then heated to 95° C. under stirring, and was held for 3 hours at 95° C.

The slurry was then washed with water by a decantation method and the pH value of the slurry was adjusted to 10.5. When the concentration of the slurry at this point was checked so as to ensure the accuracy, it was 96 g/liter.

2 liter of the slurry washed with water was filtered through a Buchner filter, and the pure water was passed until the electric conductivity of the filtrate became not more than 30 μs. The particles were then dried by an ordinary method and pulverized so as to obtain the target spindle-shaped hematite particles. The spindle-shaped hematite particles obtained contained 0.91 wt % of aluminum (calculated as Al) uniformly within the particles, and had an average major axial diameter of not more than 0.128 μm, a minor axial diameter of 0.0206 μm, a specific ratio of 6.21, a geometric standard deviation σg of particle size (major axial diameter) of 1.35, a BET specific surface (S_(BET)) of 47.1 m² /g, a S_(BET) /S_(TEM) value of densification of 1.17 and a pH value of the particles of 9.1. The spindle-shaped hematite particles contained soluble sodium salts of 112 ppm soluble sodium (calculated as Na) and soluble sulfates of 41 ppm of soluble sulfate (calculated as SO₄). The resin adsorptivity thereof was 86.3%.

Example 124

<Production of a non-magnetic undercoat layer>

12 g of the spindle-shaped hematite particles containing 0.91 wt % of aluminum (calculated as Al) uniformly within the particles obtained were mixed with a binder resin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 wt % of cyclohexanone) and cyclohexanone, and the mixture (solid content: 72 wt %) obtained was kneaded by a plasto-mill for 30 minutes.

A coating film composition was obtained in the same way as in Example 2 by using the kneaded material obtained.

The viscosity of the coating film composition obtained was 435 cP. The coating film composition obtained containing hematite particles was applied to a polyethylene terephthalate film of 12 μm thick to a thickness of 55 μm by an applicator, and the film was then dried, thereby forming a non-magnetic undercoat layer. The thickness of the non-magnetic undercoat layer was 3.4 μm. The gloss of the coating film of the non-magnetic undercoat layer was 211%, the surface roughness Ra was 6.2 nm, and the Young's modulus (relative value) was 131.

<Production of a magnetic recording layer>

12 g of acicular magnetic iron-based metal particles (average major axial diameter: 0.104 μm, average minor axial diameter: 0.0158 μm, aspect ratio: 6.58, coercive force: 1905 Oe, saturation magnetization: 138 emu/g, geometric standard deviation: 1.35, resin adsorptivity: 80.1%), which contained 1.12 wt % of aluminum in the central portion, 2.55 wt % of aluminum in the surface layer portion, and 0.48 wt % of aluminum on the surface coating (calculated as Al), respectively, and further contained 0.36 wt % of Nd, 1.2 g of a polishing agent (AKP-30: trade name, produced by Sumitomo Chemical Co., Ltd.), 0.36 g of carbon black (#3250B, trade name, produced by Mitsubishi Chemical Corp.), a binder resin solution (30 wt % of vinyl chloride-vinyl acetate copolymer resin having a sodium sulfonate group and 70 wt % of cyclohexanone) and cyclohexanone were mixed to obtain a mixture (solid content: 78 wt %). The mixture was further kneaded by a plasto-mill for 30 minutes to obtain a kneaded material.

A magnetic tape was produced in the same way as in Example 2 by using the kneaded material obtained. The thickness of the magnetic recording layer of the magnetic tape was 1.1 μm.

The magnetic tape obtained by forming the magnetic recording layer on the non-magnetic undercoat layer had a coercive force of 1981 Oe, a squareness (Br/Bm) of 0.88, a gloss of 228%, a surface roughness Ra of 6.0 nm, a Young's modulus (relative value) of 132, a linear absorption coefficient of 1.23, a running durability of 30.0 minutes, and a scratch resistance of A.

Changes in the coercive force and the saturation magnetic flux density Bm with passage time were 3.4% and 4.5%, respectively.

Examples 125 to 133, Comparative Examples 71 to 84

<Kinds of acicular goethite particles>

The following starting materials (I) to (V) were prepared as the precursors for the production of acicular hematite particles. The main producing conditions and various properties are shown in Table 20.

<Production of low-density acicular hematite particles>

Low-density acicular hematite particles were produced in the same way as in Example 123 except for varying the kind of acicular goethite particles, the kind and amount of sintering preventive, the heating temperature and time. The particle obtained in Comparative Example 74 were goethite particles.

The main producing conditions and various properties are shown in Tables 21 to 24.

Examples 134 to 142, Comparative Examples 85 to 97

<Production of high-density acicular hematite particles>

High-density acicular hematite particles were produced in the same way as in Example 123 except for varying the kind of low-density acicular hematite particles, and the heating temperature and time for densification.

The main producing conditions and various properties are shown in Tables 25 and 26.

Examples 143 to 151, Comparative Examples 98 to 105

<Treatment of acicular hematite Particles in an aqueous alkali solution>

Acicular hematite particles were obtained in the same way as in Example 123 except for varying the kind of high-density acicular hematite particles, whether or not the wet-pulverization process was conducted, whether or not the heat-treatment in the aqueous alkali solution was conducted, the pH value of the slurry, and the heating time and temperature.

The main producing conditions and various properties are shown in Tables 27 to 30.

Example 152

<Surface coating of acicular hematite particles>

The concentration of the slurry having a pH value 10.5 which was obtained in Example 143 by washing the particles in an aqueous alkali solution after heat-treatment with water by a decantation method was 96 g/liter. 5 liter of the slurry was re-heated to 60° C., and 533 ml (equivalent to 3.0 wt % (calculated as Al) based on the acicular hematite particles) of a 1.0-N NaAlO₂ solution was added to the slurry, and the mixture was held for 60 minutes. Thereafter, the pH value of the mixture was adjusted to 8.2 by using acetic acid. The particles were then filtered out, washed with water, dried and pulverized in the same way as in Example 123, thereby obtaining acicular hematite particles coated with a coating material.

The main producing conditions and various properties are shown in Tables 31 and 32.

Examples 153 to 160

Acicular hematite particles coated with a coating material were obtained in the same way as in Example 152 except for varying the kind of acicular hematite particles, and the kind and the amount of surface treating material.

The main producing conditions and various properties are shown in Tables 31 and 32.

Examples 161 to 178, Comparative Examples 106 to 120

<Production of a non-magnetic undercoat layer>

A non-magnetic undercoat layer was obtained in the same way as in Example 124 by using the acicular hematite particles obtained in Examples 143 to 160, acicular hematite particles as the starting material (V), and the acicular hematite particles obtained in Comparative Examples 73, 85 to 88, 93 and 98 to 105.

The main producing conditions and various properties are shown in Tables 33 and 34.

Examples 179 to 191, Comparative Examples 121 to 134

<Production of a magnetic recording medium>

4 kinds of magnetic iron-based metal particles (a) to (d) shown in in Table 35 were used.

A magnetic recording medium using magnetic iron-based metal particles was obtained in the same way as in Example 124 except for varying the kind of non-magnetic undercoat layer and the kind of acicular magnetic iron-based metal particles.

The main producing conditions and various properties are shown in Tables 36 and 37.

                  TABLE 1                                                          ______________________________________                                                      Precursor                                                                               Precursor                                                                               Precursor                                                                             Precursor                                Kind of precursors                                                                          1        2        3      4                                        ______________________________________                                         Production of acicular                                                         goethite particles                                                             Production method                                                                           B        B        D      C                                        Kind of Al added                                                                            Aluminum Aluminum Aluminum                                                                              Aluminum                                              sulfate  sulfate  nitrate                                                                               acetate                                  Acicular goethite                                                              particles                                                                      Average major axial                                                                         0.179    0.228    0.246  0.196                                    diameter (μm)                                                               Aspect ratio (-)                                                                            7.6      7.9      8.1    7.7                                      Geometric standard                                                                          1.35     1.32     1.30   1.38                                     deviation σg (-)                                                         BET specific 146.0    101.0    85.3   95.1                                     surface area (m.sup.2 /g)                                                      Al content (wt %)                                                                           0.60     1.12     0.84   0.20                                     Soluble Na salt                                                                             389      453      1389   365                                      (ppm)                                                                          Soluble sulfate                                                                             235      564      2323   890                                      (ppm)                                                                          pH value of  7.9      7.3      6.1    5.5                                      particles (-)                                                                  ______________________________________                                                      Precursor                                                                               Precursor                                                                               Precursor                                                                             Precursor                                Kind of precursors                                                                          5        6        7      8                                        ______________________________________                                         Production of acicular                                                         goethite particles                                                             Production method                                                                           B        B        A      B                                        Kind of Al added                                                                            Aluminum Aluminum Sodium Aluminum                                              sulfate  sulfate  aluminate                                                                             sulfate                                  Acicular goethite                                                              particles                                                                      Average major axial                                                                         0.150    0.235    0.216  0.258                                    diameter (μm)                                                               Aspect ratio (-)                                                                            7.4      8.4      8.8    8.6                                      Geometric standard                                                                          1.42     1.30     1.40   1.35                                     deviation σg (-)                                                         BET specific 186.4    65.6     75.1   60.6                                     surface area (m.sup.2 /g)                                                      Al content (wt %)                                                                           0.46     2.89     4.05   0.005                                    Soluble Na salt                                                                             456      399      1189   325                                      (ppm)                                                                          Soluble sulfate                                                                             367      412      268    525                                      (ppm)                                                                          pH value of  7.1      6.8      8.4    7.0                                      particles (-)                                                                  ______________________________________                                          (Note) PRODUCTION METHOD:                                                      (A): A method of oxidizing a suspension having a pH value of not less tha      11 and containing colloidal ferrous hydroxide particles which is obtained      by adding not less than an equivalent of an alkali hydroxide solution to       an aqueous ferrous salt solution, by passing an oxygencontaining gas           thereinto at a temperature of not higher than 80° C.                    (B): A method of producing spindleshaped goethite particles by oxidizing       suspension containing FeCO.sub.3 which is obtained by reacting an aqueous      ferrous salt solution with an aoueous alkali carbonate solution, by            passing an oxygencontaining gas thereinto after aging the suspension, if       necessary.                                                                     (C): A method of growing acicular seed goethite particles by oxidizing a       ferrous hydroxide solution containing colloidal ferrous hydroxide              particles which is obtained by adding less than an equivalent of an alkal      hydroxide solution or an alkali carbonate solution to an aqueous ferrous       salt solution, by passing an oxygencontaining gas thereinto, thereby           producing acicular seed goethite particles, adding not less than an            equivalent of an alkali hydroxide solution to the  #Fe.sup.2+  in the          aqueous ferrous salt solution, to the aqueous ferrous salt solution            containing the acicular goethite seed particles, and passing an                oxygencontaining gas into the aqueous ferrous salt solution.                   (D): A method of growing acicular seed goethite particles by oxidizing a       ferrous hydroxide solution containing colloidal ferrous hydroxide              particles which is obtained by adding less than an equivalent of an alkal      hydroxide solution or an alkali carbonate solution to an aqueous ferrous       salt solution, by passing an oxygencontaining gas thereinto, thereby           producing acicular seed goethite particles, and growing the obtained           acicular seed goethite particles in an acidic or neutral region.         

                  TABLE 2                                                          ______________________________________                                                  Example 3 Example 4 Example 5                                                                              Example 6                                 ______________________________________                                         Kind of  Precursors                                                                               Precursor Precursor 1                                                                            Precursor                                 acicular in        1                 2                                         goethite Examples                                                              particles                                                                      Sintering                                                                      preventive                                                                     Kind     #3 Water  hexameta- #3 Water                                                                               Phosphoric                                         glass     phosphate glass/ phos-                                                                           acid                                                         Soda      phoric acid                                       Amount   SiO.sub.2 : 0.75                                                                         P: 0.60   SiO.sub.2 : 1.25/                                                                      P: 1.50                                   added (wt %)                 P: 1.00                                           Heating and                                                                    dehydration                                                                    Temperature                                                                             300       350       380     350                                       (° C.)                                                                  Time (min.)                                                                             60        60        60      60                                        ______________________________________                                                  Example 7 Example 8 Example 9                                                                              Example 10                                ______________________________________                                         Kind of  Precursor 2                                                                              Precursor 3                                                                              Precursor 3                                                                            Precursor 4                               acicular                                                                       goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind     Sodium    hexameta- Boric acid                                                                             #3 Water                                           aluminate phosphate         glass                                                        Soda                                                        Amount   Al: 3.00  P: 1.20   B: 1.60 SiO.sub.2 : 1.00                          added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             330       330       350     320                                       (° C.)                                                                  Time (min.)                                                                             120       120       90      60                                        ______________________________________                                                            Example                                                              Example 11                                                                               12        Example 13                                                                             Example 14                                ______________________________________                                         Kind of  Precursor Precursor Precursor 5                                                                            Precursor                                 acicular 4         5                 6                                         goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind     Phosphoric                                                                               hexameta- #3 Water                                                                               #3 Water                                           acid      phosphate glass/  glass                                                        Soda      hexameta-                                                                      phosphate                                                                      soda                                              Amount   P: 1.00   P: 7.00   SiO.sub.2 : 0.75/                                                                      SiO.sub.2 : 1.50                          added (wt %)                 P: 1.25                                           Heating and                                                                    dehydration                                                                    Temperature                                                                             300       380       350     350                                       (° C.)                                                                  Time (min.)                                                                             30        120       90      60                                        ______________________________________                                                   Example 15   Example 16 Example 17                                   ______________________________________                                         Kind of   Precursor 6  Precursor  Precursor                                    acicular               7          7                                            goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind      Titanyl sulfate/                                                                            hexameta-  Alurninurn                                             Phosphoric acid                                                                             phosphate  sulfate                                                             Soda                                                    Amount    Ti: 3.35     P: 1.00    Al: 3.25                                     added (wt %)                                                                             P: 2.20                                                              Heating and                                                                    dehydration                                                                    Temperature                                                                              375          310        330                                          (° C.)                                                                  Time (min.)                                                                              60           30         60                                           ______________________________________                                    

                  TABLE 3                                                          ______________________________________                                                      Example 3                                                                               Example 4                                                                               Example 5                                                                             Example 6                                ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.115    0.133    0.140  0.166                                    diameter (μm)                                                               Average minor axial                                                                         0.0178   0.0218   0.0221 0.0263                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.33     1.33   1.37                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.46     6.10     6.33   6.31                                     S.sub.BET (m.sup.2 /g)                                                                      187.3    150.5    160.8  143.9                                    S.sub.TEM (m.sup.2 /g)                                                                      46.6     38.2     37.6   31.6                                     S.sub.BET /S.sub.TEM (-)                                                                    4.02     3.94     4.28   4.56                                     Al content (wt %)                                                                           0.67     0.67     0.67   1.25                                     Amount of sintering                                                                         SiO.sub.2 : 0.83                                                                        P: 0.66  SiO.sub.2 : 1.38                                                                      P: 1.60                                  preventive (wt %)              P: 1.10                                         Soluble Na salt                                                                             1361     1897     1835   1768                                     (ppm)                                                                          Soluble sulfate                                                                             568      1321     1189   1443                                     (ppm)                                                                          pH value of  7.9      7.1      7.3    7.0                                      particles (-)                                                                  ______________________________________                                                                               Example                                               Example 7                                                                               Example 8                                                                               Example 9                                                                             10                                       ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.168    0.204    0.206  0.147                                    diameter (μm)                                                               Average minor axial                                                                         0.0260   0.0291   0.0290 0.0242                                   diameter (μm)                                                               Geometric standard                                                                          1.36     1.41     1.40   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.46     7.01     7.10   6.07                                     S.sub.BET (m.sup.2 /g)                                                                      134.8    125.9    145.0  145.9                                    S.sub.TEM (m.sup.2 /g)                                                                      31.9     28.3     28.4   34.4                                     S.sub.BET /S.sub.TEM (-)                                                                    4.23     4.45     5.11   4.24                                     Al content (wt %)                                                                           1.25     0.93     0.93   0.22                                     Amount of sintering                                                                         Al: 3.24 P: 1.30  B: 1.75                                                                               SiO.sub.2 : 1.11                         preventive (wt %)                                                              Soluble Na salt                                                                             1689     2567     3109   1123                                     (ppm)                                                                          Soluble sulfate                                                                             1567     1011     980    760                                      (ppm)                                                                          pH value of  6.8      7.9      8.3    7.8                                      particles (-)                                                                  ______________________________________                                                      Example  Example  Example                                                                               Example                                               11       12       13     14                                       ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.147    0.107    0.097  0.190                                    diameter (μm)                                                               Average minor axial                                                                         0.0240   0.0181   0.0191 0.0287                                   diameter (μm)                                                               Geometric standard                                                                          1.36     1.41     1.44   1.32                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.13     5.91     5.08   6.62                                     S.sub.BET (m.sup.2 /g)                                                                      156.9    257.5    246.2  98.8                                     S.sub.TEM (m.sup.2 /g)                                                                      34.7     46.1     44.2   28.8                                     S.sub.BET /S.sub.TEM (-)                                                                    4.53     5.59     5.57   3.43                                     Al content (wt %)                                                                           0.22     0.51     0.52   3.22                                     Amount of sintering                                                                         P: 1.09  P: 7.28  SiO.sub.2 : 0.83                                                                      SiO.sub.2 : 1.68                         preventive (wt %)              P:1.36                                          Soluble Na salt                                                                             1324     1324     1145   876                                      (ppm)                                                                          Soluble sulfate                                                                             689      1126     1123   888                                      (ppm)                                                                          pH value of  8.0      6.5      6.9    6.0                                      particles (-)                                                                  ______________________________________                                                     Example 15 Example 16 Example 17                                   ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                        0.192      0.176      0.177                                        diameter (μm)                                                               Average minor axial                                                                        0.0278     0.0251     0.0255                                       diameter (μm)                                                               Geometric standard                                                                         1.32       1.36       1.37                                         deviation σg (-)                                                         Aspect ratio (-)                                                                           6.91       7.01       6.94                                         S.sub.BET (m.sup.2 /g)                                                                     101.8      126.8      136.9                                        S.sub.TEM (m.sup.2 /g)                                                                     29.7       32.8       32.3                                         S.sub.BET /S.sub.TEM (-)                                                                   3.43       3.86       4.23                                         Al content (wt %)                                                                          3.22       4.50       4.51                                         Amount of sintering                                                                        Ti: 3.55   P: 1.11    Al: 3.58                                     preventive (wt %)                                                                          P:2.41                                                             Soluble Na salt                                                                            769        1452       1562                                         (ppm)                                                                          Soluble sulfate                                                                            658        467        576                                          (ppm)                                                                          pH value of 6.1        8.1        8.0                                          particles (-)                                                                  ______________________________________                                    

                  TABLE 4                                                          ______________________________________                                                  Comp.     Comp.     Comp.   Comp.                                              Example 1 Example 2 Example 3                                                                              Example 4                                 ______________________________________                                         Kind of  Precursors                                                                               Precursors                                                                               Precursors                                                                             Precursors                                acicular in Examples                                                                              in Examples                                                                              in Examples                                                                            in Examples                               goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind     --        --        #3 Water                                                                               Phosphoric                                                             glass   acid                                      Amount   --        --        SiO.sub.2 : 1.00                                                                       P: 1.00                                   added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             320       350       350     --                                        (° C.)                                                                  Time (min.)                                                                             60        60        60      --                                        ______________________________________                                                  Comp.     Comp.     Comp.   Comp.                                              Example 5 Example 6 Example 7                                                                              Example 8                                 ______________________________________                                         Kind of  Precursors                                                                               Precursors                                                                               Precursors                                                                             Precursors                                acicular in Examples                                                                              in Examples                                                                              in Examples                                                                            in Examples                               goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind     Phosphoric                                                                               Phosphoric                                                                               #3 Water                                                                               #3 Water                                           acid      acid      glass   glass                                     Amount   P: 1.25   P: 1.00   SiO.sub.2 : 1.50                                                                       SiO.sub.2 : 1.00                          added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             330       310       320     350                                       (° C.)                                                                  Time (min.)                                                                             30        30        90      60                                        ______________________________________                                                  Comp.     Comp.     Comp.   Comp.                                              Example 9 Example 10                                                                               Example 11                                                                             Example 12                                ______________________________________                                         Kind of  Precursors                                                                               Precursor 6                                                                              Precursor 6                                                                            Precursor 6                               acicular in Examples                                                           goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind     Phosphoric                                                                               hexameta- #3 Water                                                                               Sodium                                             acid      phosphate glass   aluminate                                                    Soda                                                        Amount   P: 1.00   P: 2.00   SiO.sub.2 : 1.75                                                                       Al: 1.50                                  added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             300       380       350     330                                       (° C.)                                                                  Time (min.)                                                                             30        90        90      30                                        ______________________________________                                                    Comp.      Comp.      Comp.                                                    Example 13 Example 14 Example 15                                    ______________________________________                                         Kind of    Precursor 6                                                                               Precursor 6                                                                               Precursor 8                                   acicular                                                                       goethite                                                                       particles                                                                      Sintering                                                                      preventive                                                                     Kind       Titanyl    Phosphoric Phosphoric                                               sulfate    acid       acid                                          Amount     Ti: 1.00   P: 1.00    P: 1.50                                       added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                               325        330        350                                           (° C.)                                                                  Time (min.)                                                                               45         60         90                                            ______________________________________                                    

                  TABLE 5                                                          ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example 1                                                                               Example 2                                                                               Example 3                                                                             Example 4                                ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.114.check mark.                                                                       0.113    0.115  --                                       diameter (μm)                                                               Average minor axial                                                                         0.0178   0.0177   0.0176 --                                       diameter (μm)                                                               Geometric standard                                                                          1.35     1.33     1.33   --                                       deviation σg (-)                                                         Aspect ratio (-)                                                                            6.40     6.38     6.53   --                                       S.sub.BET (m.sup.2 /g)                                                                      187.3    150.5    160.8  --                                       S.sub.TEM (m.sup.2 /g)                                                                      46.6     46.9     47.1   --                                       S.sub.BET /S.sub.TEM (-)                                                                    4.02     3.21     3.42   --                                       Al content (wt %)                                                                           0.67     0.67     0.67   --                                       Amount of sintering                                                                         --       --       SiO.sub.2 : 1.09                                                                      P: 1.10                                  preventive (wt %)                                                              Soluble Na salt                                                                             980      889      1256   --                                       (ppm)                                                                          Soluble sulfate                                                                             635      589      789    --                                       (ppm)                                                                          pH value of  7.8      6.8      6.8    --                                       particles (-)                                                                  Resin adsorptivity                                                                          15.6     --       17.6   --                                       (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example 5                                                                               Example 6                                                                               Example 7                                                                             Example 8                                ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.116    0.115    0.117  0.114                                    diameter (μm)                                                               Average minor axial                                                                         0.0175   0.0176   0.0177 0.0178                                   diameter (μm)                                                               Geometric standard                                                                          1.34     1.35     1.35   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.63     6.53     6.61   6.40                                     S.sub.BET (m.sup.2 /g)                                                                      134.8    125.9    145.0  145.9                                    S.sub.TEM (m.sup.2 /g)                                                                      47.3     47.1     46.7   46.6                                     S.sub.BET /S.sub.TEM (-)                                                                    2.85     2.68     3.10   3.13                                     Al content (wt %)                                                                           0.67     0.68     0.67   0.67                                     Amount of sintering                                                                         P: 1.36  P: 1.10  SiO.sub.2 : 1.65                                                                      SiO.sub.2 : 1.11                         preventive (wt %)                                                              Soluble Na salt                                                                             1156     999      1678   1123                                     (ppm)                                                                          Soluble sulfate                                                                             675      567      768    760                                      (ppm)                                                                          pH value of  6.3      7.1      7.7    7.3                                      particles (-)                                                                  Resin adsorptivity                                                                          --       --       --     --                                       (%)                                                                            ______________________________________                                                               Comp.    Comp.  Comp.                                                 Comp.    Example  Example                                                                               Example                                               Example 9                                                                               10       11     12                                       ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                         0.116    0.197    0.195  0.190                                    diameter (μm)                                                               Average minor axial                                                                         0.0174   0.0288   0.0289 0.0291                                   diameter (μm)                                                               Geometric standard                                                                          1.36     1.33     1.33   1.32                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.67     6.84     6.75   6.53                                     S.sub.BET (m.sup.2 /g)                                                                      156.9    257.5    246.2  98.8                                     S.sub.TEM (m.sup.2 /g)                                                                      47.5     28.7     28.6   28.5                                     S.sub.BET /S.sub.TEM (-)                                                                    3.30     8.98     8.61   3.47                                     Al content (wt %)                                                                           0.66     3.22     3.23   3.21                                     Amount of sintering                                                                         P: 1.09  P: 2.21  SiO.sub.2 : 1.80                                                                      Al: 1.65                                 preventive (wt %)                                                              Soluble Na salt                                                                             1022     1324     1145   876                                      (ppm)                                                                          Soluble sulfate                                                                             689      675      548    888                                      (ppm)                                                                          pH value of  7.2      7.5      7.2    6.0                                      particles (-)                                                                  Resin adsorptivity                                                                          --       --       --     --                                       (%)                                                                            ______________________________________                                                       Comp.     Comp.     Comp.                                                      Example   Example   Example                                                    13        14        15                                           ______________________________________                                         Low-density hematite                                                           particles                                                                      Average major axial                                                                          0.192     0.197.check mark.                                                                        0.211                                        diameter (μm)                                                               Average minor axial                                                                          0.0298    0.0277    0.0293                                       diameter (μm)                                                               Geometric standard                                                                           1.33      1.32      1.36                                         deviation σg (-)                                                         Aspect ratio (-)                                                                             6.44      7.11      7.20                                         S.sub.BET (m.sup.2 /g)                                                                       101.8     136.9     126.6                                        S.sub.TEM (m.sup.2 /g)                                                                       27.8      29.7      28.1                                         S.sub.BET /S.sub.TEM (-)                                                                     3.66      4.61      4.51                                         Al content (wt %)                                                                            3.20      3.23      0.006                                        Amount of sintering                                                                          Ti: 1.10  P: 1.09   P: 1.63                                      preventive (wt %)                                                              Soluble Na salt                                                                              1361      1897      1835                                         (ppm)                                                                          Soluble sulfate                                                                              2343      576       872                                          (ppm)                                                                          pH value of   5.3       8.0       6.8                                          particles (-)                                                                  Resin adsorptivity                                                                           --        --        --                                           (%)                                                                            ______________________________________                                    

                  TABLE 6                                                          ______________________________________                                                      Example  Example  Example                                                                               Example                                               18       19       20     21                                       ______________________________________                                         Kind of low-density                                                                         Example  Example  Example                                                                               Example                                  acicular hematite                                                                           3        4        5      6                                        particles                                                                      Densification                                                                  Temperature (° C.)                                                                   700      700      680    730                                      Time (min.)  30       30       60     15                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.115    0.132    0.138  0.165                                    diameter (μm)                                                               Average minor axial                                                                         0.0179   0.0220   0.0223 0.0263                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.34     1.34   1.37                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.42     6.00     6.19   6.27                                     S.sub.BET (m.sup.2 /g)                                                                      58.3     51.1     48.9   40.7                                     S.sub.TEM (m.sup.2 /g)                                                                      46.3     37.9     37.3   31.6                                     S.sub.BET /S.sub.TEM (-)                                                                    1.26     1.35     1.31   1.29                                     Al content (wt %)                                                                           0.67     0.67     0.67   1.25                                     Amount of sintering                                                                         SiO.sub.2 : 0.84                                                                        P: 0.67  SiO.sub.2 : 1.38                                                                      P: 1.61                                  preventive (wt %)              P: 1.10                                         Soluble Na salt                                                                             1633     2466     2018   1968                                     (ppm)                                                                          Soluble sulfate                                                                             3378     3963     3448   3607                                     (ppm)                                                                          pH value of  5.3      5.7      5.8    5.1                                      particles (-)                                                                  ______________________________________                                                      Example  Example  Example                                                                               Example                                               22       23       24     25                                       ______________________________________                                         Kind of low-density                                                                         Example  Example  Example                                                                               Example                                  acicular hematite                                                                           7        8        9      10                                       particles                                                                      Densification                                                                  Temperature (° C.)                                                                   650      700      690    700                                      Time (min.)  60       45       30     60                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.168    0.200    0.204  0.145                                    diameter (μm)                                                               Average minor axial                                                                         0.0261   0.0294   0.0291 0.0244                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.42     1.41   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.44     6.80     7.01   5.94                                     S.sub.BET (m.sup.2 /g)                                                                      45.3     41.8     42.5   51.0                                     S.sub.TEM (m.sup.2 /g)                                                                      31.8     28.1     28.3   34.2                                     S.sub.BET /S.sub.TEM (-)                                                                    1.43     1.49     1.50   1.49                                     Al content (wt %)                                                                           1.25     0.93     0.93   0.22                                     Amount of sintering                                                                         Al: 3.23 P: 1.29  B: 1.78                                                                               SiO.sub.2 : 1.12                         preventive (wt %)                                                              Soluble Na salt                                                                             1890     2879     3330   1356                                     (ppm)                                                                          Soluble sulfate                                                                             3129     3330     2980   3103                                     (ppm)                                                                          pH value of  5.3      6.1      6.2    5.2                                      particles (-)                                                                  ______________________________________                                                      Example  Example  Example                                                                               Example                                               26       27       28     29                                       ______________________________________                                         Kind of low-density                                                                         Example  Example  Example                                                                               Example                                  acicular hematite                                                                           11       12       13     14                                       particles                                                                      Densification                                                                  Temperature (° C.)                                                                   650      700      750    700                                      Time (min.)  60       45       15     30                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.146    0.107    0.095  0.188                                    diameter (μm)                                                               Average minor axial                                                                         0.0244   0.0184   0.0193 0.0288                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.41     1.45   1.33                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            5.98     5.82     4.92   6.53                                     S.sub.BET (m.sup.2 /g)                                                                      50.1     57.5     51.5   40.1                                     S.sub.TEM (m.sup.2 /g)                                                                      34.2     45.4     43.9   28.8                                     S.sub.BET /S.sub.TEM (-)                                                                    1.47     1.27     1.17   1.39                                     Al content (wt %)                                                                           0.22     0.51     0.52   3.22                                     Amount of sintering                                                                         P: 1.10  P: 7.28  SiO.sub.2 : 0.85                                                                      SiO.sub.2 : 1.69                         preventive (wt %)              P: 1.35                                         Soluble Na salt                                                                             1546     1678     1329   1022                                     (ppm)                                                                          Soluble sulfate                                                                             2980     3789     3671   3223                                     (ppm)                                                                          pH value of  5.4      5.0      5.3    5.0                                      particles (-)                                                                  ______________________________________                                                       Example   Example    Example                                                   30        31         32                                          ______________________________________                                         Kind of low-density                                                                          Example   Example    Example                                     acicular hematite                                                                            15        16         17                                          particles                                                                      Densification                                                                  Temperature (° C.)                                                                    700       650        680                                         Time (min.)   30        60         60                                          High-density acicular                                                          hematite particles                                                             Average major axial                                                                          0.190     0.175      0.175                                       diameter (μm)                                                               Average minor axial                                                                          0.0281    0.0253     0.0255                                      diameter (μm)                                                               Geometric standard                                                                           1.32      1.36       1.38                                        deviation σg (-)                                                         Aspect ratio (-)                                                                             6.76      6.92       6.86                                        S.sub.BET (m.sup.2 /g)                                                                       40.5      46.4       45.9                                        S.sub.TEM (m.sup.2 /g)                                                                       29.4      32.6       32.4                                        S.sub.BET /S.sub.TEM (-)                                                                     1.38      1.42       1.42                                        Al content (wt %)                                                                            3.22      4.50       4.51                                        Amount of sintering                                                                          Ti: 3.55  P: 1.13    Al: 3.61                                    preventive (wt %)                                                                            P: 2.44                                                          Soluble Na salt                                                                              1129      1658       1659                                        (ppm)                                                                          Soluble sulfate                                                                              4895      769        1345                                        (ppm)                                                                          pH value of   4.8       7.8        7.1                                         particles (-)                                                                  ______________________________________                                    

                  TABLE 7                                                          ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               16       17       18     19                                       ______________________________________                                         Kind of low-density                                                                         Comp.    Comp.    Comp.  Comp.                                    acicular hematite                                                                           Example  Example  Example                                                                               Example                                  particles    1        2        4      5                                        Densification                                                                  Temperature (° C.)                                                                   700      650      690    710                                      Time (min.)  15       15       30     60                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.076    0.086    0.121  0.115                                    diameter (μm)                                                               Average minor axial                                                                         0.0321   0.0256   0.0198 0.0176                                   diameter (μm)                                                               Geometric standard                                                                          1.96     1.71     1.56   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            2.37     3.36     6.11   6.53                                     S.sub.BET (m.sup.2 /g)                                                                      11.5     21.9     39.6   53.2                                     S.sub.TEM (m.sup.2 /g)                                                                      29.0     34.5     42.0   47.1                                     S.sub.BET /S.sub.TEM (-)                                                                    0.40     0.63     0.94   1.13                                     Al content (wt %)                                                                           0.67     0.67     0.67   0.67                                     Amount of sintering                                                                         --       --       P: 1.09                                                                               P: 1.36                                  preventive (wt %)                                                              Soluble Na salt                                                                             1754     1845     1657   1489                                     (ppm)                                                                          Soluble sulfate                                                                             3157     3765     3890   3678                                     (ppm)                                                                          pH value of  5.5      5.6      5.2    5.1                                      particles (-)                                                                  Resin adsorptivity                                                                          12.6     18.8     14.4   21.6                                     (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               20       21       22     23                                       ______________________________________                                         Kind of low-density                                                                         Comp.    Comp.    Comp.  Comp.                                    acicular hematite                                                                           Example  Example  Example                                                                               Exainple                                 particles    6        7        8      9                                        Densification                                                                  Temperature (° C.)                                                                   560      720      730    520                                      Time (min.)  90       45       10     60                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.116    0.116    0.115  0.116                                    diameter (μm)                                                               Average minor axial                                                                         0.0175   0.0177   0.0177 0.0174                                   diameter (μm)                                                               Geometric standard                                                                          1.34     1.35     1.35   1.34                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.63     6.55     6.50   6.67                                     S.sub.BET (m.sup.2 /g)                                                                      61.2     53.9     58.5   73.5                                     S.sub.TEM (m.sup.2 /g)                                                                      47.3     46.8     46.8   47.5                                     S.sub.BET /S.sub.TEM (-)                                                                    1.29     1.15     1.25   1.55                                     Al content (wt %)                                                                           0.68     0.67     0.67   0.66                                     Amount of sintering                                                                         P: 1.12  SiO.sub.2 : 1.63                                                                        SiO.sub.2 : 1.12                                                                      P: 1.11                                  preventive (wt %)                                                              Soluble Na salt                                                                             1580     1462     1765   1487                                     (ppm)                                                                          Soluble sulfate                                                                             3217     3649     3795   3098                                     (ppm)                                                                          pH value of  5.1      5.5      5.6    5.6                                      particles (-)                                                                  Resin adsorptivity                                                                          --       --       --     --                                       (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               24       25       26     27                                       ______________________________________                                         Kind of low-density                                                                         Comp.    Comp.    Comp.  Comp.                                    acicular hematite                                                                           Example  Example  Example                                                                               Example                                  particles    10       11       12     13                                       Densification                                                                  Temperature (° C.)                                                                   650      650      600    750                                      Time (min.)  60       45       15     30                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.195    0.195    0.188  0.190                                    diameter (μm)                                                               Average minor axial                                                                         0.0289   0.0292   0.0295 0.0300                                   diameter (μm)                                                               Geometric standard                                                                          1.34     1.33     1.32   1.33                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.75     6.68     6.37   6.33                                     S.sub.BET (m.sup.2 /g)                                                                      41.0     43.9     51.5   40.1                                     S.sub.TEM (m.sup.2 /g)                                                                      28.6     28.3     28.1   27.7                                     S.sub.BET /S.sub.TEM (-)                                                                    1.43     1.55     1.83   1.45                                     Al content (wt %)                                                                           3.22     3.23     3.21   3.20                                     Amount of sintering                                                                         P: 2.25  SiO.sub.2 : 1.83                                                                        Al: 1.63                                                                              Ti: 1.10                                 preventive (wt %)                                                              Soluble Na salt                                                                             2167     2156     1256   1190                                     (ppm)                                                                          Soluble sulfate                                                                             3679     3264     3690   4678                                     (ppm)                                                                          pH value of  6.1      6.0      5.2    4.9                                      particles (-)                                                                  Resin adsorptivity                                                                          36.5     --       --     --                                       (%)                                                                            ______________________________________                                                           Comp.       Comp.                                                              Example     Example                                                            28          29                                               ______________________________________                                         Kind of low-density                                                                              Comp.       Comp.                                            acicular hematite Example     Example                                          particles         14          15                                               Densification                                                                  Temperature (° C.)                                                                        450         680                                              Time (min.)       30          30                                               High-density acicular                                                          hematite particles                                                             Average major axial                                                                              0.198       0.206                                            diameter (μm)                                                               Average minor axial                                                                              0.0278      0.0299                                           diameter (μm)                                                               Geometric standard                                                                               1.32        1.37                                             deviation σg (-)                                                         Aspect ratio (-)  7.12        6.89                                             S.sub.BET (m.sup.2 /g)                                                                           73.0        38.7                                             S.sub.TEM (m.sup.2 /g)                                                                           29.6        27.6                                             S.sub.BET /S.sub.TEM (-)                                                                         2.47        1.40                                             Al content (wt %) 3.23        0.006                                            Amount of sintering                                                                              P: 1.10     P: 1.65                                          preventive (wt %)                                                              Soluble Na salt   2456        1280                                             (ppm)                                                                          Soluble sulfate   3356        3103                                             (ppm)                                                                          pH value of       5.9         5.6                                              particles (-)                                                                  Resin adsorptivity                                                                               --          --                                               (%)                                                                            ______________________________________                                    

                  TABLE 8                                                          ______________________________________                                                    Exam-   Exam-   Exam-                                                          ple     ple     ple   Example                                                                               Example                                           33      34      35    36     37                                     ______________________________________                                         Kind of high-                                                                             Exam-   Exam-   Exam- Example                                                                               Example                                density acicular                                                                          ple     ple     ple   21     22                                     hematite particles                                                                        18      19      20                                                  Wet pulverization                                                              Yes or No  Yes     Yes     Yes   Yes    Yes                                    Amount of residue                                                                         0       0       0     0      0                                      on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.8    13.5    13.6  13.4   13.1                                   Temperature (° C.)                                                                 98      95      93    90     97                                     Time (min. )                                                                              180     210     180   180    120                                    ______________________________________                                                    Exam-   Exam-   Exam-                                                          ple     ple     ple   Example                                                                               Example                                           38      39      40    41     42                                     ______________________________________                                         Kind of high-                                                                             Exam-   Exam-   Exam- Example                                                                               Example                                density acicular                                                                          ple     ple     ple   26     27                                     hematite particles                                                                        23      24      25                                                  Wet pulverization                                                              Yes or No  Yes     Yes     Yes   Yes    Yes                                    Amount of residue                                                                         0       0       0     0      0                                      on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.8    13.7    13.4  13.7   13.5                                   Temperature (° C.)                                                                 95      93      93    95     93                                     Time (min.)                                                                               90      120     180   180    140                                    ______________________________________                                                    Exam-   Exam-   Exam-                                                          ple     ple     ple   Example                                                                               Example                                           43      44      45    46     47                                     ______________________________________                                         Kind of high-                                                                             Exam-   Exam-   Exam- Example                                                                               Example                                density acicular                                                                          ple     ple     ple   31     32                                     hematite particles                                                                        28      29      30                                                  Wet pulverization                                                              Yes or No  Yes     Yes     Yes   Yes    Yes                                    Amount of residue                                                                         0       0       0     0      0                                      on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.5    13.1    13.1  13.3   13.7                                   Temperature (° C.)                                                                 98      97      90    91     95                                     Time (min.)                                                                               120     180     90    180    180                                    ______________________________________                                    

                  TABLE 9                                                          ______________________________________                                                      Example  Example  Example                                                                               Example                                               33       34       35     36                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.115    0.133    0.137  0.166                                    diameter (μm)                                                               Average minor axial                                                                         0.0179   0.0219   0.0223 0.0262                                   diameter (μm)                                                               Geometric standard                                                                          1.34     1.35     1.34   1.37                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.42     6.02     6.14   6.34                                     S.sub.BET (m.sup.2 /g)                                                                      57.4     52.2     49.0   41.2                                     S.sub.TEM (m.sup.2 /g)                                                                      46.3     38.0     37.3   31.7                                     S.sub.BET /S.sub.TEM (-)                                                                    1.24     1.37     1.31   1.30                                     Al content (wt %)                                                                           0.67     0.67     0.67   1.25                                     Amount of sintering                                                                         SiO.sub.2 : 0.84                                                                        P: 0.38  SiO.sub.2 : 1.37                                                                      P: 0.86                                  preventive (wt %)              P: 0.55                                         Soluble Na salt                                                                             111      121      98     134                                      (ppm)                                                                          Soluble sulfate                                                                             12       15       21     11                                       (ppm)                                                                          pH value of  9.1      9.3      8.9    9.5                                      particles (-)                                                                  Resin adsorptivity                                                                          78.8     80.6     83.6   71.2                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               37       38       39     40                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.168    0.199    0.202  0.144                                    diameter (μm)                                                               Average minor axial                                                                         0.0261   0.0293   0.0291 0.0244                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.41     1.41   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.44     6.79     6.94   5.90                                     S.sub.BET (m.sup.2 /g)                                                                      45.5     41.9     43.1   51.9                                     S.sub.TEM (m.sup.2 /g)                                                                      31.8     28.2     28.3   34.2                                     S.sub.BET /S.sub.TEM (-)                                                                    1.43     1.49     1.52   1.52                                     Al content (wt %)                                                                           1.25     0.93     0.93   0.22                                     Amount of sintering                                                                         Al: 3.23 P: 0.61  B: 0.24                                                                               SiO.sub.2 : 1.13                         preventive (wt %)                                                              Soluble Na salt                                                                             142      110      108    89                                       (ppm)                                                                          Soluble sulfate                                                                             12       21       16     9                                        (ppm)                                                                          pH value of  9.6      9.2      9.0    8.8                                      particles (-)                                                                  Resin adsorptivity                                                                          78.1     88.9     85.5   80.1                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               41       42       43     44                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.145    0.106    0.095  0.188                                    diameter (μm)                                                               Average minor axial                                                                         0.0244   0.0183   0.0193 0.0287                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.41     1.45   1.33                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            5.94     5.79     4.92   6.55                                     S.sub.BET (m.sup.2 /g)                                                                      50.5     58.0     52.3   40.3                                     S.sub.TEM (m.sup.2 /g)                                                                      34.2     45.7     43.9   28.8                                     S.sub.BET /S.sub.TEM (-)                                                                    1.48     1.27     1.19   1.40                                     Al content (wt %)                                                                           0.22     0.51     0.52   3.22                                     Amount of sintering                                                                         P: 0.36  P: 2.89  SiO.sub.2 : 0.83                                                                      SiO.sub.2 : 1.68                         preventive (wt %)              P: 0.71                                         Soluble Na salt                                                                             98       124      68     96                                       (ppm)                                                                          Soluble sulfate                                                                             16       34       46     21                                       (ppm)                                                                          pH value of  8.8      9.3      9.0    8.9                                      particles (-)                                                                  Resin adsorptivity                                                                          79.5     76.8     71.1   84.3                                     (%)                                                                            ______________________________________                                                       Example   Example   Example                                                    45        46        47                                           ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                          0.189     0.175     0.174                                        diameter (μm)                                                               Average minor axial                                                                          0.0281    0.0253    0.0255                                       diameter (μm)                                                               Geometric standard                                                                           1.32      1.35      1.37                                         deviation σg (-)                                                         Aspect ratio (-)                                                                             6.73      6.92      6.82                                         S.sub.BET (m.sup.2 /g)                                                                       41.1      46.9      44.9                                         S.sub.TEM (m.sup.2 /g)                                                                       29.4      32.6      32.4                                         S.sub.BET /S.sub.TEM (-)                                                                     1.40      1.44      1.39                                         Al content (wt %)                                                                            3.22      4.50      4.51                                         Amount of sintering                                                                          Ti: 3.54  P: 0.56   Al: 3.60                                     preventive (wt %)                                                                            P: 1.13                                                          Soluble Na salt                                                                              87        129       100                                          (ppm)                                                                          Soluble sulfate                                                                              16        15        5                                            (ppm)                                                                          pH value of   9.2       9.3       9.2                                          particles (-)                                                                  Resin adsorptivity                                                                           80.8      71.9      84.6                                         (%)                                                                            ______________________________________                                    

                  TABLE 10                                                         ______________________________________                                                    Comp.   Comp.   Comp.                                                          Exam-   Exam-   Exam- Comp.  Comp.                                             ple     ple     ple   Example                                                                               Example                                           30      31      32    33     34                                     ______________________________________                                         Kind of acicular                                                                          Comp.   Comp.   Comp. Comp.  Comp.                                  hematite particles                                                                        Exam-   Exam-   Exam- Example                                                                               Example                                           ple     ple     ple   23     25                                                20      21      22                                                  Wet pulverization                                                              Yes or No  Yes     Yes     Yes   No     No                                     Amount of residue                                                                         0       0       0     18.6   29.5                                   on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              --      12.1    13.2  13.2   10.3                                   Temperature (° C.)                                                                 --      95      68    90     97                                     Time (min.)                                                                               --      180     180   180    180                                    ______________________________________                                                    Comp.     Comp.    Comp.   Comp.                                               Example   Example  Example Example                                             35        36       37      38                                       ______________________________________                                         Kind of acicular                                                                          Comp.     Comp.    Comp.   Comp.                                    hematite particles                                                                        Example   Example  Example Example                                             26        27       28      29                                       Wet pulverization                                                              Yes or No  No        No       Yes     Yes                                      Amount of residue                                                                         30.4      21.6     0       0                                        on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.5      13.4     9.3     13.6                                     Temperature (° C.)                                                                 95        93       95      93                                       Time (min.)                                                                               180       120      120     180                                      ______________________________________                                    

                  TABLE 11                                                         ______________________________________                                                        Comp.     Comp.     Comp.                                                      Example   Example   Example                                                    30        31        32                                          ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                           0.115     0.116     0.115                                       diameter (μm)                                                               Average minor axial                                                                           0.175     0.0176    0.0177                                      diameter (μm)                                                               Geometric standard                                                                            1.34      1.35      1.34                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.57      6.59      6.50                                        S.sub.BET (m.sup.2 /g)                                                                        62.0      53.8      58.4                                        S.sub.TEM (m.sup.2 /g)                                                                        47.3      47.0      46.8                                        S.sub.BET /S.sub.TEM (-)                                                                      1.31      1.14      1.25                                        Al content (wt %)                                                                             0.68      0.67      0.67                                        Amount of sintering                                                                           P: 1.12   SiO.sub.2 : 1.62                                                                         SiO.sub.2 : 1.11                            preventive (wt %)                                                              Soluble Na salt                                                                               712       413       378                                         (ppm)                                                                          Soluble sulfate                                                                               436       336       168                                         (ppm)                                                                          pH value of    6.7       7.0       8.2                                         particles (-)                                                                  Resin adsorptivity                                                                            64.1      59.3      64.6                                        (%)                                                                            ______________________________________                                                        Comp.     Comp.     Comp.                                                      Example   Example   Example                                                    33        34        35                                          ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                           0.117     0.198     0.192                                       diameter (μm)                                                               Average minor axial                                                                           0.0177    0.0294    0.0297                                      diameter (μm)                                                               Geometric standard                                                                            1.37      1.37      1.41                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.61      6.73      6.46                                        S.sub.BET (m.sup.2 /g)                                                                        73.9      43.0      51.9                                        S.sub.TEM (m.sup.2 /g)                                                                        46.7      28.1      27.9                                        S.sub.BET /S.sub.TEM (-)                                                                      1.58      1.53      1.86                                        Al content (wt %)                                                                             0.66      3.23      3.21                                        Amount of sintering                                                                           P: 0.61   SiO.sub.2 : 1.80                                                                         Al: 1.65                                    preventive (wt %)                                                              Soluble Na salt                                                                               490       698       468                                         (ppm)                                                                          Soluble sulfate                                                                               225       289       265                                         (ppm)                                                                          pH value of    7.3       7.4       7.0                                         particles (-)                                                                  Resin adsorptivity                                                                            53.1      46.5      41.6                                        (%)                                                                            ______________________________________                                                        Comp.     Comp.     Comp.                                                      Example   Example   Example                                                    36        37        38                                          ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                           0.191     0.197     0.206                                       diameter (μm)                                                               Average minor axial                                                                           0.0302    0.0274    0.0300                                      diameter (μm)                                                               Geometric standard                                                                            1.41      1.37      1.35                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.32      7.19      6.87                                        S.sub.BET (m.sup.2 /g)                                                                        40.7      74.8      37.8                                        S.sub.TEM (m.sup.2 /g)                                                                        27.5      30.0      27.5                                        S.sub.BET /S.sub.TEM (-)                                                                      1.48      2.49      1.37                                        Al content (wt %)                                                                             3.20      3.23      0.006                                       Amount of sintering                                                                           Ti: 1.00  P: 0.96   P: 0.61                                     preventive (wt %)                                                              Soluble Na salt                                                                               435       549       116                                         (ppm)                                                                          Soluble sulfate                                                                               214       310       56                                          (ppm)                                                                          pH value of    7.5       7.1       8.8                                         particles (-)                                                                  Resin adsorptivity                                                                            58.6      60.1      63.6                                        (%)                                                                            ______________________________________                                    

                  TABLE 12                                                         ______________________________________                                                      Example  Example  Example                                                                               Example                                               48       49       50     51                                       ______________________________________                                         Kind of acicular                                                                            Example  Example  Example                                                                               Example                                  hematite particles                                                                          33       34       35     36                                       treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         Sodium   #3 Water Aluminum                                                                              Colloidal                                             aluminate                                                                               glass    sulfate                                                                               silica                                   Amount added (wt %)                                                                         5.0      1.0      1.5    3.0                                      Coating material                                                               Kind         Al*      Si*      Al*    Si*                                      Amount (wt %)                                                                               4.76     0.98     1.47   2.90                                     ______________________________________                                                      Example 52    Example 53                                          ______________________________________                                         Kind of acicular                                                                            Example 37    Example 38                                          hematite particles                                                             treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         Aluminum acetate/                                                                            Aluminum sulfate/                                                #3 Water glass                                                                               #3 Water glass                                      Amount added (wt %)                                                                         3.0/1.0       0.5/3.0                                             Coating material                                                               Kind         Al*/Si*       Al*/Si*                                             Amount (wt %)                                                                               2.91/0.97     0.49/2.86                                           ______________________________________                                                     Example                Example                                                 54        Example 55   56                                          ______________________________________                                         Kind of acicular                                                                           Example   Example 40   Example                                     hematite particles                                                                         39                     41                                          treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind        Sodium    Sodium aluminate/                                                                           Sodium                                                  aluminate Colloidal silica                                                                            aluminate                                   Amount added (wt %)                                                                        10.       1.5/2.0      0.5                                         Coating material                                                               Kind        Al*       Al*/Si*      Al*                                         Amount (wt %)                                                                              9.09      1.48/1.96    0.49                                        ______________________________________                                                     Example   Example                                                              57        58       Example 59                                      ______________________________________                                         Kind of acicular                                                                           Example   Example  Example 45                                      hematite particles                                                                         44        43                                                       treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind        Aluminum  #3 Water Sodium aluminate/                                           acetate   glass    Aluminum acetate                                Amount added (wt %)                                                                        15.0      5.0      2.0/4.0                                         Coating material                                                               Kind        Al*       Si*      Al*                                             Amount (wt %)                                                                              13.05     4.75     5.80                                            ______________________________________                                                                  Example   Example                                                  Example 60  61        62                                          ______________________________________                                         Kind of acicular                                                                            Example 45  Example   Example                                     hematite particles       46        47                                          treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         #3 Water glass/                                                                            Sodium    Sodium                                                   Colloidal silica                                                                           aluminate aluminate                                   Amount added (wt %)                                                                         0.2/0.3     7.5       20.0                                        Coating material                                                               Kind         Si*         Al*       Al*                                         Amount (wt %)                                                                               0.46        6.96      16.64                                       ______________________________________                                          Al*: aluminum hydroxide, Si*: silicon oxide                              

                  TABLE 13                                                         ______________________________________                                                      Example  Example  Example                                                                               Example                                               48       49       50     51                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                         0.115    0.133    0.137  0.165                                    diameter (μm)                                                               Average minor axial                                                                         0.0179   0.0220   0.0222 0.0262                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.35     1.34   1.36                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.42     6.05     6.17   6.30                                     S.sub.BET (m.sup.2 /g)                                                                      57.2     54.0     47.8   43.6                                     S.sub.TEM (m.sup.2 /g)                                                                      46.3     37.9     37.5   31.7                                     S.sub.BET /S.sub.TEM (-)                                                                    1.23     1.43     1.28   1.38                                     Al content (wt %)                                                                           0.67     0.67     0.67   1.25                                     Amount of sintering                                                                         SiO.sub.2 : 0.81                                                                        P: 0.37  SiO.sub.2 : 1.35                                                                      P: 0.81                                  preventive (wt %)              P: 0.52                                         Soluble Na salt                                                                             123      112      89     125                                      (ppm)                                                                          Soluble sulfate                                                                             19       23       43     20                                       (ppm)                                                                          pH value of  9.2      9.1      9.4    9.0                                      particles (-)                                                                  Resin adsorptivity                                                                          80.1     81.5     80.8   88.3                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               52       53       54     55                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                         0.167    0.200    0.201  0.144                                    diameter (μm)                                                               Average minor axial                                                                         0.0261   0.0293   0.0292 0.0244                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.42     1.41   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.40     6.83     6.88   5.90                                     S.sub.BET (m.sup.2 /g)                                                                      46.6     43.9     43.9   53.8                                     S.sub.TEM (m.sup.2 /g)                                                                      31.8     28.2     28.3   34.2                                     S.sub.BET /S.sub.TEM (-)                                                                    1.47     1.56     1.55   1.57                                     Al content (wt %)                                                                           1.25     0.93     0.93   0.22                                     Amount of sintering                                                                         Al: 3.19 P: 0.56  B: 0.15                                                                               SiO.sub.2 : 1.10                         preventive (wt %)                                                              Soluble Na salt                                                                             138      144      100    134                                      (ppm)                                                                          Soluble sulfate                                                                             8        49       11     12                                       (ppm)                                                                          pH value of  9.5      8.9      9.3    9.3                                      particles (-)                                                                  Resin adsorptivity                                                                          81.9     93.6     94.2   88.9                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               56       57       58     59                                       ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                         0.144    0.106    0.096  0.189                                    diameter (μm)                                                               Average minor axial                                                                         0.0244   0.0183   0.0193 0.0288                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.41     1.42   1.33                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            5.90     5.79     4.97   6.56                                     S.sub.BET (m.sup.2 /g)                                                                      50.0     56.8     54.1   38.9                                     S.sub.TEM (m.sup.2 /g)                                                                      34.2     45.7     43.9   28.7                                     S.sub.BET /S.sub.TEM (-)                                                                    1.46     1.24     1.23   1.35                                     Al content (wt %)                                                                           0.22     0.51     0.52   3.22                                     Amount of sintering                                                                         P: 0.36  P: 2.66  SiO.sub.2 : 0.80                                                                      SiO.sub.2 : 1.58                         preventive (wt %)              P: 0.65                                         Soluble Na salt                                                                             53       96       115    88                                       (ppm)                                                                          Soluble sulfate                                                                             23       37       21     15                                       (ppm)                                                                          pH value of  8.8      8.9      9.0    9.1                                      particles (-)                                                                  Resin adsorptivity                                                                          85.6     77.6     75.6   90.6                                     (%)                                                                            ______________________________________                                                       Example   Example   Example                                                    60        61        62                                           ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                          0.189     0.175     0.175                                        diameter (μm)                                                               Average minor axial                                                                          0.0282    0.0253    0.0254                                       diameter (μm)                                                               Geometric standard                                                                           1.33      1.36      1.36                                         deviation σg (-)                                                         Aspect ratio (-)                                                                             6.70      6.92      6.89                                         S.sub.BET (m.sup.2 /g)                                                                       43.6      47.4      48.1                                         S.sub.TEM (m.sup.2 /g)                                                                       29.3      32.6      32.5                                         S.sub.BET /S.sub.TEM (-)                                                                     1.49      1.45      1.48                                         Al content (wt %)                                                                            3.22      4.50      4.51                                         Amount of sintering                                                                          Ti: 3.32  P: 0.51   Al: 3.16                                     preventive (wt %)                                                                            P: 1.10                                                          Soluble Na salt                                                                              129       76        102                                          (ppm)                                                                          Soluble sulfate                                                                              13        8         23                                           (ppm)                                                                          pH value of   9.6       9.2       9.1                                          particles (-)                                                                  Resin adsorptivity                                                                           83.6      81.0      91.6                                         (%)                                                                            ______________________________________                                    

                  TABLE 14                                                         ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          63      64      65     66     67                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     35     36     37                                     particles 33      34                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           435     410     563    384    410                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.4     3.4     3.5    3.5    3.4                                    Gloss (%) 206     200     198    198    196                                    Ra (nm)   6.8     7.2     7.5    7.0    8.0                                    Young's modulus                                                                          121     122     124    127    129                                    (-)                                                                            (relative value)                                                               ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          68      69      70     71     72                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     40     41     42                                     particles 38      39                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0.                                   particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           205     230     435    410    845                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.6     3.2     3.4    3.5    3.8                                    Gloss (%) 191     193     205    202    211                                    Ra (nm)   8.6     8.8     7.0    6.8    6.0                                    Young's modulus                                                                          131     135     118    119    116                                    (-)                                                                            (relative value)                                                               ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          73      74      75     76     77                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     45     46     47                                     particles 43      44                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           896     230     230    205    205                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.7     3.3     3.4    3.2    3.3                                    Gloss (%) 214     195     197    199    195                                    Ra (nm)   6.3     6.8     7.0    6.8    6.7                                    Young's modulus                                                                          118     128     126    122    125                                    (-)                                                                            (relative value)                                                               ______________________________________                                    

                  TABLE 15                                                         ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          78      79      80     81     82                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     50     51     52                                     particles 48      49                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           384     435     435    307    384                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.3     3.3     3.4    3.3    3.4                                    Gloss (%) 216     206     202    205    200                                    Ra (nm)   6.0     6.6     6.8    6.4    7.0                                    Young's modulus                                                                          124     124     126    129    131                                    (-)                                                                            (relative value)                                                               ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          83      84      85     86     87                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     55     56     57                                     particles 53      54                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           154     128     384    333    742                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.5     3.5     3.4    3.3    3.5                                    Gloss (%) 196     196     206    206    216                                    Ra (nm)   7.5     7.9     6.8    6.2    5.4                                    Young's modulus                                                                          135     136     121    123    120                                    (-)                                                                            (relative value)                                                               ______________________________________                                                   Exam-   Exam-                                                                  ple     ple     Example                                                                               Example                                                                               Example                                          88      89      90     91     92                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Exam-   Exam-   Example                                                                               Example                                                                               Example                                hematite  ple     ple     60     61     62                                     particles 58      59                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particies and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           712     179     154    179    179                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.5     3.3     3.4    3.3    3.3                                    Gloss (%) 218     198     198    201    199                                    Ra (nm)   5.9     6.0     6.3    6.1    6.3                                    Young's modulus                                                                          121     130     128    123    130                                    (-)                                                                            (relative value)                                                               ______________________________________                                    

                  TABLE 16                                                         ______________________________________                                                   Comp.   Comp.                                                                  Exam-   Exam-   Comp.  Comp.  Comp.                                            ple     ple     Example                                                                               Example                                                                               Example                                          39      40      41     42     43                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Comp.   Comp.   Comp.  Comp.  Coinp.                                 hematite  Exam-   Exam-   Example                                                                               Example                                                                               Example                                particles ple     ple     17     3      18                                               1       16                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           12800   230     333    11776  563                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.8     3.2     3.3    4.0    3.7                                    Gloss (%) 56      34      78     80     148                                    Ra (nm)   84.0    116.0   56.7   46.8   31.7                                   Young's modulus                                                                          84      76      88     97     103                                    (-)                                                                            (relative value)                                                               ______________________________________                                                   Comp.   Comp.                                                                  Exam-   Exam-   Comp.  Comp.  Comp.                                            ple     ple     Example                                                                               Example                                                                               Example                                          44      45      46     47     48                                     ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                         Comp.   Comp.   Comp.  Comp.  Comp.                                  hematite  Exam-   Exam-   Example                                                                               Example                                                                               Example                                particles ple     ple     31     32     33                                               19      30                                                           Weight ratio of                                                                          5.0     5.0     5.0    5.0    5.0                                    particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                           435     563     435    384    512                                    Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                        3.5     3.7     3.3    3.5    3.7                                    Gloss (%) 156     168     175    182    164                                    Ra (nm)   28.5    16.9    15.2   13.1   18.8                                   Young's modulus                                                                          101     106     101    107    103                                    (-)                                                                            (relative value)                                                               ______________________________________                                                  Comp.   Comp.   Comp. Comp. Comp. Comp.                                        Exam-   Exam-   Exam- Exam- Exam- Exam-                                        ple     ple     ple   ple   ple   ple                                          49      50      51    52    53    54                                  ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of  Comp.   Comp.   Comp. Comp. Comp. Comp.                               acicular Exam-   Exam-   Exam- Exam- Exam- Exam-                               hematite ple     ple     ple   ple   ple   ple                                 particles                                                                               24      34      35    36    37    38                                  Weight ratio of                                                                         5.0     5.0     5.0   5.0   5.0   5.0                                 particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                          410     614     666   742   410   333                                 Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                       3.7     3.6     3.8   3.4   3.5   3.2                                 Gloss (%)                                                                               160     146     139   148   166   198                                 Ra (nm)  20.2    25.5    38.6  34.5  20.4  8.0                                 Young's  109     106     107   109   112   128                                 modulus (-)                                                                    (relative value)                                                               ______________________________________                                    

                  TABLE 17                                                         ______________________________________                                                      Ex. 93  Ex. 94  Ex. 95                                                                               Ex. 96                                                                               Ex. 97                                ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 63  Ex. 64  Ex. 65                                                                               Ex. 66                                                                               Ex. 67                                undercoat layer                                                                Kind of magnetic iron-                                                                      (I)     (I)     (I)   (I)   (I)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.2     1.2   1.1   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1940    1938    1931  1949  1930                                  Br/Bm (-)    0.88    0.88    0.89  0.88  0.88                                  Gloss (%)    239     235     233   235   230                                   Ra (nm)      6.4     6.8     6.9   6.4   6.9                                   Young's modulus (-)                                                                         133     134     136   140   140                                   (relative value)                                                               Durability                                                                     Running durability                                                                          22.6    26.7    22.3  28.9  26.4                                  (min)                                                                          Scratch resistance                                                                          A       A       B     A     A                                     Linear absorption                                                                           1.21    1.25    1.24  1.30  1.29                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     4.6     5.7     8.5   6.5   7.4                                   coercive force (%)                                                             Percentage of change in                                                                     6.8     7.9     7.4   6.9   5.7                                   Bm (%)                                                                         ______________________________________                                                                      Ex.   Ex.   Ex.                                                Ex. 98  Ex. 99  100   101   102                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 68  Ex. 69  Ex. 70                                                                               Ex. 71                                                                               Ex. 72                                undercoat layer                                                                Kind of magnetic iron-                                                                      (I)     (I)     (II)  (II)  (II)                                  based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.2     1.3   1.2   1..1                                  layer (μm)                                                                  Coercive force (Oe)                                                                         1910    1934    1710  1714  1725                                  Br/Bm (-)    0.89    0.88    0.87  0.87  0.88                                  Gloss (%)    225     221     228   229   238                                   Ra (nm)      7.3     7.3     6.8   6.5   5.3                                   Young's modulus (-)                                                                         144     146     128   131   128                                   (relative value)                                                               Durability                                                                     Running durability                                                                          30≦                                                                             30≦                                                                             22.5  24.5  18.6                                  (min)                                                                          Scratch resistance                                                                          A       A       A     A     B                                     Linear absorption                                                                           1.33    1.35    1.41  1.43  1.21                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     6.5     6.9     7.8   4.6   6.8                                   coercive force (%)                                                             Percentage of change in                                                                     4.8     4.9     6.8   7.2   7.8                                   Bm (%)                                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                103     104     105   106   107                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 73  Ex. 74  Ex. 75                                                                               Ex. 76                                                                               Ex. 77                                undercoat layer                                                                Kind of magnetic iron-                                                                      (II)    (II)    (II)  (II)  (II)                                  based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.1     1.2   1.2   1.3                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1690    1730    1723  1698  1702                                  Br/Bm (-)    0.88    0.89    0.88  0.88  0.87                                  Gloss (%)    238     219     222   227   220                                   Ra (nm)      5.6     6.4     6.5   6.0   6.5                                   Young's modulus (-)                                                                         130     138     138   133   136                                   (relative value)                                                               Durability                                                                     Running durability                                                                          16.8    28.9    29.0  25.7  27.1                                  (min)                                                                          Scratch resistance                                                                          B       A       A     B     A                                     Linear absorption                                                                           1.23    1.25    1.24  1.24  1.22                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     7.1     8.8     3.6   4.7   4.7                                   coercive force (%)                                                             Percentage of change in                                                                     8.9     6.4     7.0   5.7   8.7                                   Bm (%)                                                                         ______________________________________                                          (I): Major axial diameter = 0.11 μm; Minor axial diameter = 0.018           μm; Aspect ratio = 6.1; Hc = 1880 Oe; σs = 128 emu/g; pH value =      9.9.                                                                           (II): Major axial diameter = 0.14 μm; Minor axial diameter = 0.021          μm; Aspect ratio = 6.7; Hc = 1650 Oe; σs = 134 emu/g; pH value =      10.0.                                                                    

                  TABLE 18                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                108     109     110   111   112                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 78  Ex. 79  Ex. 80                                                                               Ex. 81                                                                               Ex. 82                                undercoat layer                                                                Kind of magnetic iron-                                                                      (I)     (I)     (I)   (I)   (I)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.1     1.2   1.1   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1954    1945    1940  1950  1.949                                 Br/Bm (-)    0.89    0.88    0.89  0.89  0.89                                  Gloss (%)    241     238     240   237   236                                   Ra (nm)      6.0     6.4     6.3   6.4   6.3                                   Young's modulus (-)                                                                         136     135     139   141   144                                   (relative value)                                                               Durability                                                                     Running durability                                                                          28.9    29.9    27.5  30≦                                                                           28.8                                  (min)                                                                          Scratch resistance                                                                          A       A       A     A     A                                     Linear absorption                                                                           1.22    1.26    1.27  1.32  1.32                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     3.2     4.6     5.3   3.8   7.1                                   coercive force (%)                                                             Percentage of change in                                                                     4.8     6.0     5.6   5.1   4.3                                   Bm (%)                                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                113     114     115   116   117                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 83  Ex. 84  Ex. 85                                                                               Ex. 86                                                                               Ex. 87                                undercoat layer                                                                Kind of magnetic iron-                                                                      (I)     (I)     (II)  (II)  (II)                                  based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.2     1.1   1.2   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1946    1956    1723  1734  1734                                  Br/Bm (-)    0.88    0.90    0.88  0.88  0.88                                  Gloss (%)    228     226     229   231   234                                   Ra (nm)      6.8     6.8     6.0   6.0   5.4                                   Young's modulus (-)                                                                         146     147     133   133   132                                   (relative value)                                                               Durability                                                                     Running durability                                                                          30≦                                                                             30≦                                                                             30≦                                                                           30≦                                                                           20.7                                  (min)                                                                          Scratch resistance                                                                          A       A       A     A     A                                     Linear absorption                                                                           1.33    1.36    1.43  1.44  1.25                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     6.1     5.0     6.3   3.5   4.0                                   coercive force (%)                                                             Percentage of change in                                                                     3.7     4.0     5.1   4.7   5.2                                   Bm (%)                                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                118     119     120   121   122                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex. 88  Ex. 89  Ex. 90                                                                               Ex. 91                                                                               Ex. 92                                undercoat layer                                                                Kind of magnetic iron-                                                                      (II)    (II)    (II)  (II)  (II)                                  based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.1     1.2   1.1   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1708    1723    1735  1713  1732                                  Br/Bm (-)    0.89    0.89    0.88  0.88  0.89                                  Gloss (%)    237     227     230   231   225                                   Ra (nm)      5.4     6.1     6.3   6.4   6.2                                   Young's modulus (-)                                                                         133     138     141   135   138                                   (relative value)                                                               Durability                                                                     Running durability                                                                          22.8    28.7    27.8  29.5  25.8                                  (min)                                                                          Scratch resistance                                                                          A       A       A     A     A                                     Linear absorption                                                                           1.24    1.25    1.24  1.23  1.25                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     3.7     6.9     2.8   4.1   3.5                                   coercive force (%)                                                             Percentage of change in                                                                     6.1     5.8     5.9   4.3   3.6                                   Bm (%)                                                                         ______________________________________                                          (I): Major axial diameter = 0.11 μm; Minor axial diameter = 0.018           μm; Aspect ratio = 6.1; Hc = 1880 Oe; σs = 128 emu/g; pH value =      9.9.                                                                           (II): Major axial diameter = 0.14 μm; Minor axial diameter = 0.021          μm; Aspect ratio = 6.7; Hc = 1650 Oe; σs = 134 emu/g; pH value =      10.0.                                                                    

                  TABLE 19                                                         ______________________________________                                                        Comp.    Comp.   Comp.  Comp.                                                  Ex. 55   Ex. 56  Ex. 57 Ex. 58                                  ______________________________________                                         Magnetic recording                                                             medium using magnetic iron-                                                    based metal particles                                                          Kind of non-magnetic                                                                          Comp.    Comp.   Comp.  Comp.                                   undercoat layer                                                                               Ex. 39   Ex. 40  Ex. 41 Ex. 42                                  Kind of magnetic iron-                                                                        (I)      (I)     (I)    (I)                                     based metal particles                                                          Weight ratio of                                                                               5.0      5.0     5.0    5.0                                     magnetic particies and                                                         resin (-)                                                                      Thickness of magnetic                                                                         1.3      1.2     1.2    1.2                                     layer (μm)                                                                  Coercive force (Oe)                                                                           1890     1880    1895   1901                                    Br/Bm (-)      0.82     0.78    0.83   0.83                                    Gloss (%)      146      112     156    161                                     Ra (nm)        64.0     78.8    42.6   32.4                                    Young's modulus (-)                                                                           93       87      99     107                                     (relative value)                                                               Durability                                                                     Running durability                                                                            0.8      1.2     0.4    4.1                                     (min)                                                                          Scratch resistance                                                                            D        C       D      C                                       Linear absorption                                                                             0.70     0.72    0.84   0.91                                    coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                       45.6     38.9    49.0   28.6                                    coercive force (%)                                                             Percentage of change in                                                                       25.7     25.1    36.8   24.5                                    Bm (%)                                                                         ______________________________________                                                        Comp.    Comp.   Comp.  Comp.                                                  Ex. 59   Ex. 60  Ex. 61 Ex. 62                                  ______________________________________                                         Magnetic recording                                                             medium using magnetic iron-                                                    based metal particles                                                          Kind of non-magnetic                                                                          Comp.    Comp.   Comp.  Comp.                                   undercoat layer                                                                               Ex. 43   Ex. 44  Ex. 451                                                                               Ex. 46                                  Kind of magnetic iron-                                                                        (I)      (I)     (I)    (I)                                     based metal particles                                                          Weight ratio of                                                                               5.0      5.0     5.0    5.0                                     magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                         1.2      1.3     1.1    1.3                                     layer (μm)                                                                  Coercive force (Oe)                                                                           1911     1905    1918   1914                                    Br/Bm (-)      0.84     0.84    0.85   0.83                                    Gloss (%)      176      186     189    193                                     Ra (nm)        25.8     24.8    15.1   11..9                                   Young's modulus (-)                                                                           113      111     117    114                                     (relative value)                                                               Durability                                                                     Running durability                                                                            6.0      3.6     9.8    10.3                                    (min)                                                                          Scratch resistance                                                                            C        C       C      B                                       Linear absorption                                                                             1.03     1.06    1.12   1.08                                    coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                       23.6     31.3    18.0   14.7                                    coercive force (%)                                                             Percentage of change in                                                                       28.9     23.1    18.8   14.3                                    Bm (%)                                                                         ______________________________________                                                        Comp.    Comp.   Comp.  Comp.                                                  Ex. 63   Ex. 64  Ex. 65 Ex. 66                                  ______________________________________                                         Magnetic recording                                                             medium using magnetic iron-                                                    based metal particles                                                          Kind of non-magnetic                                                                          Comp.    Comp.   Comp.  Comp.                                   undercoat layer                                                                               Ex. 47   Ex. 48  Ex. 49 Ex. 50                                  Kind of magnetic iron-                                                                        (I)      (I)     (I)    (I)                                     based metal particles                                                          Weight ratio of                                                                               5.0      5.0     5.0    5.0                                     magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                         1.1      1.2     1.1    1.0                                     layer (μm)                                                                  Coercive force (Oe)                                                                           1903     1905    1896   1906                                    Br/Bm (-)      0.85     0.84    0.83   0.82                                    Gloss (%)      193      178     167    165                                     Ra (nm)        13.1     15.6    21.6   21.6                                    Young's modulus (-)                                                                           121      131     123    117                                     (relative value)                                                               Durability                                                                     Running durability                                                                            8.9      8.6     5.8    4.3                                     (min)                                                                          Scratch resistance                                                                            C        C       C      D                                       Linear absorption                                                                             1.00     1.18    1.20   1.21                                    coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                       16.8     17.9    36.9   21.7                                    coercive force (%)                                                             Percentage of change in                                                                       15.3     17.9    23.9   18.0                                    Bm (%)                                                                         ______________________________________                                                        Comp.    Comp.   Comp.  Comp.                                                  Ex. 67   Ex. 68  Ex. 69 Ex. 70                                  ______________________________________                                         Magnetic recording                                                             medium using magnetic iron-                                                    based metal particles                                                          Kind of non-magnetic                                                                          Comp.    Comp.   Comp.  Comp.                                   undercoat layer                                                                               Ex. 51   Ex. 52  Ex. 53 Ex. 54                                  Kind of magnetic iron-                                                                        (I)      (I)     (I)    (I)                                     based metal particles                                                          Weight ratio of                                                                               5.0      5.0     5.0    5.0                                     magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                         1.3      1.3     1.2    1.1                                     layer (μm)                                                                  Coercive force (Oe)                                                                           1893     1899    1913   1925                                    Br/Bm (-)      0.84     0.85    0.85   0.88                                    Gloss (%)      150      163     171    225                                     Ra (nm)        28.0     26.3    12.8   7.8                                     Young's modulus (-)                                                                           123      123     124    139                                     (relative value)                                                               Durability                                                                     Running durability                                                                            3.8      8.8     9.9    9.4                                     (min)                                                                          Scratch resistance                                                                            D        C       C      B                                       Linear absorption                                                                             1.21     1.15    1.20   1.29                                    coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                       16.8     18.6    16.0   8.6                                     coercive force (%)                                                             Percentage of change in                                                                       21.0     16.9    13.8   6.2                                     Bm (%)                                                                         ______________________________________                                          (I): Major axial diameter = 0.11 μm; Minor axial diameter = 0.018           μm; Aspect ratio = 6.1; Hc = 1880 Oe; σs = 138 emu/g; pH value =      9.9.                                                                     

                  TABLE 20                                                         ______________________________________                                                        Starting  Starting  Starting                                                   material  material  material                                                   (I)       (II)      (III)                                       ______________________________________                                         Production of acicular                                                         goethite particles                                                             Production method                                                                             (B)       (C)       (D)                                         Kind of Al added                                                                              Aluminum  Aluminum  Aluminum                                                   sulfate   acetate   nitrate                                     Acicular goethite                                                              particles                                                                      Average major axial                                                                           0.146     0.178     0.135                                       diameter (μm)                                                               Average minor axial                                                                           0.0195    0.0225    0.0187                                      diameter (μm)                                                               Aspect ratio (-)                                                                              7.49      7.91      7.22                                        Geometric standard                                                                            1.36      1.45      1.30                                        deviation σg (-)                                                         BET specific surface                                                                          198.4     175.4     225.9                                       area (m.sup.2 /g)                                                              Al content (wt %)                                                                             2.31      1.23      4.56                                        Soluble Na salt (ppm)                                                                         415       568       1325                                        Soluble sulfate (ppm)                                                                         359       543       1865                                        pH value of particles                                                                         6.8       7.2       5.2                                         (-)                                                                            ______________________________________                                                           Starting    Starting                                                           material    material                                                           (IV)        (V)                                              ______________________________________                                         Production of acicular                                                         goethite particles                                                             Production method (A)         (C)                                              Kind of Al added  Sodium                                                                         aluminate                                                    Acicular goethite                                                              particles                                                                      Average major axial                                                                              0.201       0.264                                            diameter (μm)                                                               Average minor axial                                                                              0.0236      0.0318                                           diameter (μm)                                                               Aspect ratio (-)  8.52        8.30                                             Geometric standard                                                                               1.42        1.41                                             deviation σg (-)                                                         BET specific surface                                                                             91.8        61.2                                             area (m.sup.2 /g)                                                              Al content (wt %) 0.83        0.003                                            Soluble Na salt (ppm)                                                                            1265        514                                              Soluble sulfate (ppm)                                                                            387         435                                              pH value of particles                                                                            8.6         7.1                                              (-)                                                                            ______________________________________                                          (Note) PRODUCTION METHOD:                                                      (A): A method of oxidizing a suspension having a pH value of not less tha      11 and containing colloidal ferrous hydroxide particles which is obtained      by adding not less than an equivalent of an alkali hydroxide solution to       an aqueous ferrous salt solution, by passing an oxygencontaining gas           thereinto at a temperature of not higher than 80° C.                    (B): A method of producing spindleshaped goethite particles by oxidizing       suspension containing FeCO.sub.3 which is obtained by reacting an aqueous      ferrous salt solution with an aqueous alkali carbonate solution, by            passing an oxygencontaining gas thereinto after aging the suspension, if       necessary.                                                                     (C): A method of growing acicular seed goethite particles by oxidizing a       ferrous hydroxide solution containing colloidal ferrous hydroxide              particles which is obtained by adding less than an equivalent of an alkal      hydroxide solution or an alkali carbonate solution to an aqueous ferrous       salt solution, by passing an oxygencontaining gas thereinto, thereby           producing acicular seed goethite particles, adding not less than an            equivalent of an alkali hydroxide solution to the Fe.sup.2+  in the            aqueous  #ferrous salt solution, to the aqueous ferrous salt solution          containing the acicular goethite seed particles, and passing an                oxygencontaining gas into the aqueous ferrous salt solution.                   (D): A method of growing acicular seed goethite particles by oxidizing a       ferrous hydroxide solution containing colloidal ferrous hydroxide              particles which is obtained by adding less than an equivalent of an alkal      hydroxide solution or an alkali carbonate solution to an aqueous ferrous       salt solution, by passing an oxygencontaining gas thereinto, thereby           producing acicular seed goethite particles, and growing the obtained           acicular seed goethite particles in an acidic or neutral region.         

                  TABLE 21                                                         ______________________________________                                                  Example 125 Example 126 Example 127                                   ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material in material (I)                                                                               material (I)                                  material Example 123                                                           Sintering                                                                      preventive                                                                     Kind     Hexameta-   #3 Water glass                                                                             #3 Water glass/                                        phosphate soda          phosphoric acid                               Amount   P: 0.80     SiO.sub.2 : 1.30                                                                           SiO.sub.2 : 1.00                              added (wt %)         P: 0.50                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             320         310         350                                           (° C.)                                                                  Time (min.)                                                                             45          60          45                                            ______________________________________                                                  Example 128 Example 129 Example 130                                   ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material (II)                                                                              material (II)                                                                              material (III)                                material                                                                       Sintering                                                                      preventive                                                                     Kind     #3 Water glass                                                                             Titanyl sulfate                                                                            Phosphoric acid                               Amount   SiO.sub.2 : 2.00                                                                           Ti: 3.00    P: 0.75                                       added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             330         330         300                                           (° C.)                                                                  Time (min.)                                                                             75          60          90                                            ______________________________________                                                  Example 131 Example 132 Example 133                                   ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material (III)                                                                             material (IV)                                                                              material (IV)                                 material                                                                       Sintering                                                                      preventive                                                                     Kind     Boric acid  #3 Water glass                                                                             #3 Water glass/                                                                Hexameta-                                                                      phosphate soda                                Amount   B: 0.50     SiO.sub.2 : 1.50                                                                           SiO.sub.2 : 0.50                              added (wt %)                     P: 0.75                                       Heating and                                                                    dehydration                                                                    Temperature                                                                             310         350         300                                           (° C.)                                                                  Time (min.)                                                                             75          60          60                                            ______________________________________                                    

                  TABLE 22                                                         ______________________________________                                                           Example  Example  Example                                                      125      126      127                                        ______________________________________                                         Low-density hematite particles                                                 Average major axial diameter (μm)                                                             0.135    0.120    0.123                                      Average minor axial diameter (μm)                                                             0.0191   0.0191   0.0189                                     Geometric standard deviation                                                                     1.35     1.36     1.37                                       σg (-)                                                                   Aspect ratio (-)  7.07     6.28     6.51                                       S.sub.BET (m.sup.2 /g)                                                                           178.9    211.2    225.0                                      S.sub.TEM (m.sup.2 /g)                                                                           43.1     43.5     43.8                                       S.sub.BET /S.sub.TEM (-)                                                                         4.15     4.86     5.13                                       Al content (wt %) 0.91     2.51     2.53                                       Amount of sintering                                                                              P: 0.70  SiO.sub.2 : 1.22                                                                        SiO.sub.2 : 0.94                           preventive (wt %)                   P: 0.44                                    Soluble Na salt (ppm)                                                                            1265     1077     1356                                       Soluble sulfate (ppm)                                                                            569      752      865                                        pH value of particles (-)                                                                        6.5      7.0      7.0                                        ______________________________________                                                           Example  Example  Example                                                      128      129      130                                        ______________________________________                                         Low-density hematite particles                                                 Average major axial diameter (μm)                                                             0.142    0.140    0.111                                      Average minor axial diameter (μm)                                                             0.0201   0.0206   0.0175                                     Geometric standard deviation                                                                     1.45     1.45     1.32                                       σg (-)                                                                   Aspect ratio (-)  7.06     6.80     6.34                                       S.sub.BET (m.sup.2 /g)                                                                           205.4    195.3    168.2                                      S.sub.TEM (m.sup.2 /g)                                                                           41.0     40.1     47.4                                       S.sub.BET /S.sub.TEM (-)                                                                         5.01     4.87     3.55                                       Al content (wt %) 1.35     1.31     4.98                                       Amount of sintering                                                                              SiO.sub.2 : 1.81                                                                        Ti: 3.22 P: 0.63                                    preventive (wt %)                                                              Soluble Na salt (ppm)                                                                            1546     1759     2356                                       Soluble sulfate (ppm)                                                                            1056     983      2654                                       pH value of particles (-)                                                                        6.5      7.2      6.9                                        ______________________________________                                                           Example  Example  Example                                                      131      132      133                                        ______________________________________                                         Low-density hematite particies                                                 Average major axial diameter (μm)                                                             0.108    0.164    0.168                                      Average minor axial diameter (μm)                                                             0.0177   0.0242   0.0240                                     Geometric standard deviation                                                                     1.33     1.43     1.42                                       σg (-)                                                                   Aspect ratio (-)  6.10     6.78     7.00                                       S.sub.BET (m.sup.2 /g)                                                                           175.3    138.0    146.5                                      S.sub.TEM (m.sup.2 /g)                                                                           47.0     34.1     34.3                                       S.sub.BET /S.sub.TEM (-)                                                                         3.73     4.04     4.27                                       Al content (wt %) 4.93     1.01     1.03                                       Amount of sintering                                                                              B: 0.21  SiO.sub.2 : 1.36                                                                        SiO.sub.2 : 0.44                           preventive (wt %)                   P: 0.67                                    Soluble Na salt (ppm)                                                                            2564     1236     1498                                       Soluble sulfate (ppm)                                                                            2850     854      792                                        pH value of particles (-)                                                                        6.5      7.7      7.9                                        ______________________________________                                    

                  TABLE 23                                                         ______________________________________                                                  Comp. Example                                                                              Comp. Example                                                                              Comp. Example                                          71          72          73                                            ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material (V)                                                                               material (V)                                                                               material (V)                                  material                                                                       Sintering                                                                      preventive                                                                     Kind     --          --          #3 Water glass                                Amount   --          --          SiO.sub.2 : 0.75                              added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             350         350         350                                           (° C.)                                                                  Time (min.)                                                                             60          30          60                                            ______________________________________                                                  Comp. Example                                                                              Comp. Example                                                                              Comp. Example                                          74          75          76                                            ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material (V)                                                                               material (V)                                                                               material (V)                                  material                                                                       Sintering                                                                      preventive                                                                     Kind     Phosphoric acid                                                                            Colloidal   Phosphoric acid                                                    silica                                                    Amount   P: 1.00     SiO.sub.2 : 1.25                                                                           P: 0.50                                       added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             --          320         300                                           (° C.)                                                                  Time (min.)                                                                             --          30          30                                            ______________________________________                                                  Comp. Example                                                                              Comp. Example                                                                              Comp. Example                                          77          78          79                                            ______________________________________                                         Kind of  Starting    Starting    Starting                                      starting material (V)                                                                               material (V)                                                                               material (V)                                  material                                                                       Sintering                                                                      preventive                                                                     Kind     #3 Water glass                                                                             #3 Water glass                                                                             Phosphoric acid                               Amount   SiO.sub.2 : 1.50                                                                           SiO.sub.2 : 1.00                                                                           P: 1.00                                       added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             350         350         310                                           (° C.)                                                                  Time (min.)                                                                             60          60          60                                            ______________________________________                                                  Comp. Example                                                                              Comp. Example                                                                              Comp. Example                                          80          81          82                                            ______________________________________                                         Kind of  Particles in                                                                               Particles in                                                                               Particles in                                  starting Examples 123                                                                               Examples 123                                                                               Examples 123                                  material                                                                       Sintering                                                                      preventive                                                                     Kind     Hexameta-   #3 Water glass                                                                             Boric acid                                             phosphate soda                                                        Amount   P: 1.25     SiO.sub.2 : 1.50                                                                           B: 0.75                                       added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature                                                                             350         350         330                                           (° C.)                                                                  Time (min.)                                                                             90          60          60                                            ______________________________________                                                      Comp. Example Comp. Example                                                    83            84                                                  ______________________________________                                         Kind of      Particles in  Particles in                                        starting     Examples 123  Examples 123                                        material                                                                       Sintering                                                                      preventive                                                                     Kind         Titanyl sulfate                                                                              Phosphoric acid                                     Amount       Ti: 0.50      P: 1.00                                             added (wt %)                                                                   Heating and                                                                    dehydration                                                                    Temperature  350           330                                                 (° C.)                                                                  Time (min.)  30            45                                                  ______________________________________                                    

                  TABLE 24                                                         ______________________________________                                                    Comp.    Comp.     Comp.  Comp.                                                Example  Example   Example                                                                               Example                                              71       72        73     74                                        ______________________________________                                         Low-density                                                                    hematite particles                                                             Average major                                                                             0.213    0.214     0.214  --                                        axial diameter                                                                 (μm)                                                                        Average minor                                                                             0.0298   0.0296    0.0297 --                                        axial diameter                                                                 (μm)                                                                        Geometric  1.42     1.41      1.41   --                                        standard                                                                       deviation σg (-)                                                         Aspect ratio (-)                                                                          7.15     7.23      7.21   --                                        S.sub.BET (m.sup.2 /g)                                                                    78.9     81.5      123.8  --                                        S.sub.TEM (m.sup.2 /g)                                                                    27.6     27.8      27.7   --                                        S.sub.BET /S.sub.TEM (-)                                                                  2.86     2.93      4.47   --                                        Al content (wt %)                                                                         0.003    0.003     0.003  --                                        Amount of  --       --        SiO.sub.2 : 0.70                                                                      --                                        sintering                                                                      preventive (wt %)                                                              Soluble Na salt                                                                           657      756       1189   --                                        (ppm)                                                                          Soluble sulfate                                                                           564      567       547    --                                        (ppm)                                                                          pH of particles                                                                           6.3      6.5       7.0    --                                        (-)                                                                            Resin      8.7      --        13.8   --                                        adsorptivity                                                                   (%)                                                                            ______________________________________                                                    Comp.    Comp.     Comp.  Comp.                                                Example  Example   Example                                                                               Example                                              75       76        77     78                                        ______________________________________                                         Low-density                                                                    hematite particles                                                             Average major                                                                             0.213    0.215     0.214  0.213                                     axial diameter                                                                 (μm)                                                                        Average minor                                                                             0.0296   0.0299    0.0298 0.0297                                    axial diameter                                                                 (μm)                                                                        Geometric  1.42     1.43      1.41   1.42                                      standard                                                                       deviation σg (-)                                                         Aspect ratio (-)                                                                          7.20     7.19      7.18   7.17                                      S.sub.BET (m.sup.2 /g)                                                                    131.5    125.9     135.6  123.6                                     S.sub.TEM (m.sup.2 /g)                                                                    27.8     27.5      27.6   27.7                                      S.sub.BET /S.sub.TEM (-)                                                                  4.73     4.58      4.91   4.46                                      Al content (wt %)                                                                         0.003    0.003     0.003  0.003                                     Amount of  SiO.sub.2 : 1.38                                                                        P: 0.46   SiO.sub.2 : 1.26                                                                      SiO.sub.2 : 0.89                          sintering                                                                      preventive (wt %)                                                              Soluble Na salt                                                                           1245     1345      1232   1426                                      (ppm)                                                                          Soluble sulfate                                                                           657      589       686    631                                       (ppm)                                                                          pH of particles                                                                           7.3      7.1       7.1    7.3                                       (-)                                                                            Resin      --       --        --     --                                        adsorptivity                                                                   (%)                                                                            ______________________________________                                                    Comp.    Comp.     Comp.  Comp.                                                Example  Example   Example                                                                               Example                                              79       80        81     82                                        ______________________________________                                         Low-density                                                                    hematite particles                                                             Average major                                                                             0.213    0.135     0.134  0.135                                     axial diameter                                                                 (μm)                                                                        Average minor                                                                             0.0300   0.0191    0.0190 0.0189                                    axial diameter                                                                 (μm)                                                                        Geometric  1.43     1.36      1.35   1.35                                      standard                                                                       deviation σg (-)                                                         Aspect ratio (-)                                                                          7.10     7.07      7.05   7.14                                      S.sub.BET (m.sup.2 /g)                                                                    126.5    185.9     173.6  185.0                                     S.sub.TEM (m.sup.2 /g)                                                                    27.4     43.1      43.4   43.5                                      S.sub.BET /S.sub.TEM (-)                                                                  4.61     4.31      4.00   4.25                                      Al content (wt %)                                                                         0.003    0.91      0.91   0.91                                      Amount of  P: 0.76  P: 1.16   SiO.sub.2 : 1.40                                                                      B: 0.39                                   sintering                                                                      preventive (wt %)                                                              Soluble Na salt                                                                           1127     1287      1325   1124                                      (ppm)                                                                          Soluble sulfate                                                                           568      564       678    769                                       (ppm)                                                                          pH of particles                                                                           7.3      6.9       6.8    7.0                                       (-)                                                                            Resin      --       --        --     --                                        adsorptivity                                                                   (%)                                                                            ______________________________________                                                         Comp.       Comp.                                                              Example     Example                                                            83          84                                                 ______________________________________                                         Low-density                                                                    hematite particles                                                             Average major   0.133       0.136                                              axial diameter                                                                 (μm)                                                                        Average minor   0.0199      0.0201                                             axial diameter                                                                 (μm)                                                                        Geometric       1.34        1.35                                               standard                                                                       deviation σg (-)                                                         Aspect ratio (-)                                                                               6.68        6.77                                               S.sub.BET (m.sup.2 /g)                                                                         156.9       179.2                                              S.sub.TEM (m.sup.2 /g)                                                                         41.5        41.1                                               S.sub.BET /S.sub.TEM (-)                                                                       3.78        4.36                                               Al content (wt %)                                                                              0.91        0.91                                               Amount of       Ti: 0.55    P: 0.83                                            sintering                                                                      preventive (wt %)                                                              Soluble Na salt 1546        1438                                               (ppm)                                                                          Soluble sulfate 867         687                                                (ppm)                                                                          pH of particles 6.8         7.0                                                (-)                                                                            Resin           --          --                                                 adsorptivity                                                                   (%)                                                                            ______________________________________                                    

                  TABLE 25                                                         ______________________________________                                                        Example   Example   Example                                                    134       135       136                                         ______________________________________                                         Kind of low-density                                                                           Example   Example   Example                                     acicular hematite                                                                             125       126       127                                         particles                                                                      Densification                                                                  Temperature (° C.)                                                                     650       680       630                                         Time (min.)    20        30        60                                          High-density acicular                                                          hematite particles                                                             Average major axial                                                                           0.134     0.120     0.122                                       diameter (μm)                                                               Average minor axial                                                                           0.0192    0.0193    0.0190                                      diameter (μm)                                                               Geometric standard                                                                            1.38      1.41      1.41                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.98      6.22      6.42                                        S.sub.BET (m.sup.2 /g)                                                                        48.9      53.8      54.1                                        S.sub.TEM (m.sup.2 /g)                                                                        42.9      43.1      43.6                                        S.sub.BET /S.sub.TEM (-)                                                                      1.14      1.25      1.24                                        Al content (wt %)                                                                             0.91      2.51      2.53                                        Amount of sintering                                                                           P: 0.70   SiO.sub.2 : 1.22                                                                         SiO.sub.2 : 0.95                            preventive (wt %)                  P: 0.44                                     Soluble Na salt                                                                               1633      2466      2018                                        (ppm)                                                                          Soluble sulfate                                                                               2376      2879      2345                                        (ppm)                                                                          pH value of    5.0       5.4       5.3                                         particles (-)                                                                  ______________________________________                                                        Example   Example   Example                                                    137       138       139                                         ______________________________________                                         Kind of low-density                                                                           Example   Example   Example                                     acicular hematite                                                                             128       129       130                                         particles                                                                      Densification                                                                  Temperature (° C.)                                                                     730       650       720                                         Time (min.)    15        90        30                                          High-density acicular                                                          hematite particles                                                             Average major axial                                                                           0.141     0.140     0.110                                       diameter (μm)                                                               Average minor axial                                                                           0.0203    0.0206    0.0177                                      diameter (μm)                                                               Geometric standard                                                                            1.48      1.47      1.36                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.95      6.80      6.21                                        S.sub.BET (m.sup.2 /g)                                                                        43.6      45.3      57.5                                        S.sub.TEM (m.sup.2 /g)                                                                        40.6      40.1      47.0                                        S.sub.BET /S.sub.TEM (-)                                                                      1.07      1.13      1.22                                        Al content (wt %)                                                                             1.35      1.31      4.98                                        Amount of sintering                                                                           SiO.sub.2 : 1.80                                                                         Ti: 3.20  P: 0.64                                     preventive (wt %)                                                              Soluble Na salt                                                                               1968      1890      2879                                        (ppm)                                                                          Soluble sulfate                                                                               3607      2135      4153                                        (ppm)                                                                          pH value of    5.1       5.7       4.8                                         particles (-)                                                                  ______________________________________                                                        Example   Example   Exanple                                                    140       141       142                                         ______________________________________                                         Kind of low-density                                                                           Example   Example   Example                                     acicular hematite                                                                             131       132       133                                         particles                                                                      Densification                                                                  Temperature (° C.)                                                                     700       680       650                                         Time (min.)    30        450       60                                          High-density acicular                                                          hematite particles                                                             Average major axial                                                                           0.108     0.162     0.165                                       diameter (μm)                                                               Average minor axial                                                                           0.0176    0.0242    0.0242                                      diameter (μm)                                                               Geometric standard                                                                            1.37      1.46      1.46                                        deviation σg (-)                                                         Aspect ratio (-)                                                                              6.14      6.69      6.82                                        S.sub.BET (m.sup.2 /g)                                                                        56.5      37.4      40.0                                        S.sub.TEM (m.sup.2 /g)                                                                        47.3      34.2      34.1                                        S.sub.BET /S.sub.TEM (-)                                                                      1.20      1.09      1.17                                        Al content (wt %)                                                                             4.93      1.01      1.03                                        Amount of sintering                                                                           B: 0.23   SiO.sub.2 : 1.35                                                                         SiO.sub.2 : 0.44                            preventive (wt %)                  P: 0. 68                                    Soluble Na salt                                                                               3330      1356      1546                                        (ppm)                                                                          Soluble sulfate                                                                               4789      2176      2673                                        (ppm)                                                                          pH value of    4.6       5.2       5.8                                         particles (-)                                                                  ______________________________________                                    

                  TABLE 26                                                         ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               85       86       87     88                                       ______________________________________                                         Kind of low-density                                                                         Starting Comp.    Comp.  Comp.                                    acicular hematite                                                                           material Example  Example                                                                               Example                                  particles    (V)      72       74     75                                       Densification                                                                  Temperature (° C.)                                                                   680      650      700    700                                      Time (min.)  30       30       30     45                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.086    0.136    0.159  0.212                                    diameter (μm)                                                               Average minor axial                                                                         0.0379   0.0347   0.0330 0.0310                                   diameter (μm)                                                               Geometric standard                                                                          1.96     1.71     1.56   1.51                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            2.27     3.92     4.82   6.84                                     S.sub.BET (m.sup.2 /g)                                                                      12.4     18.9     24.7   31.7                                     S.sub.TEM (m.sup.2 /g)                                                                      24.8     25.0     25.7   26.6                                     S.sub.BET /S.sub.TEM (-)                                                                    0.50     0.76     0.96   1.19                                     Al content (wt %)                                                                           0.003    0.003    0.003  0.003                                    Amount of sintering                                                                         --       --       P: 0.87                                                                               SiO.sub.2 : 1.38                         preventive (wt %)                                                              Soluble Na salt                                                                             1578     1456     1768   1546                                     (ppm)                                                                          Soluble sulfate                                                                             3246     3562     3125   2986                                     (ppm)                                                                          pH value of  5.3      5.2      5.5    5.7                                      particles (-)                                                                  Resin adsorptivity                                                                          7.4      10.4     11.5   18.5                                     (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               89       90       91     92                                       ______________________________________                                         Kind of low-density                                                                         Comp.    Comp.    Comp.  Comp.                                    acicular hematite                                                                           Example  Example  Example                                                                               Example                                  particles    76       77       78     79                                       Densification                                                                  Temperature (° C.)                                                                   560      720      700    570                                      Time (min.)  60       30       30     30                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.214    0.211    0.212  0.213                                    diameter (μm)                                                               Average minor axial                                                                         0.0297   0.0299   0.0301 0.0300                                   diameter (μm)                                                               Geometric standard                                                                          1.42     1.43     1.46   1.42                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            7.21     7.06     7.04   7.10                                     S.sub.BET (m.sup.2 /g)                                                                      44.0     33.5     36.5   43.2                                     S.sub.TEM (m.sup.2 /g)                                                                      27.7     27.5     27.4   27.5                                     S.sub.BET /S.sub.TEM (-)                                                                    1.59     1.22     1.33   1.57                                     Al content (wt %)                                                                           0.003    0.003    0.003  0.003                                    Amount of sintering                                                                         P: 0.46  SiO.sub.2 : 1.25                                                                        SiO.sub.2 : 0.90                                                                      P: 0.76                                  preventive (wt %)                                                              Soluble Na salt                                                                             1678     1539     1675   1690                                     (ppm)                                                                          Soluble sulfate                                                                             2849     3128     2870   3176                                     (ppm)                                                                          pH value of  5.5      5.0      5.5    5.4                                      particles (-)                                                                  Resin adsorptivity                                                                          --       --       --     --                                       (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp..                                                Example  Example  Example                                                                               Example                                               93       94       95     96                                       ______________________________________                                         Kind of low-density                                                                         Comp.    Comp.    Comp.  Comp.                                    acicular hematite                                                                           Example  Example  Example                                                                               Example                                  particles    80       81       82     83                                       Densification                                                                  Temperature (° C.)                                                                   680      680      600    750                                      Time (min.)  30       60       60     30                                       High-density acicular                                                          hematite particles                                                             Average major axial                                                                         0.133    0.134    0.135  0.130                                    diameter (μm)                                                               Average minor axial                                                                         0.0191   0.0192   0.0188 0.0190                                   diameter (μm)                                                               Geometric standard                                                                          1.36     1.36     1.35   1.35                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.96     6.98     7.18   6.84                                     S.sub.BET (m.sup.2 /g)                                                                      53.1     49.6     60.7   50.0                                     S.sub.TEM (m.sup.2 /g)                                                                      43.2     42.9     43.8   43.4                                     S.sub.BET /S.sub.TEM (-)                                                                    1.23     1.16     1.39   1.15                                     Al content (wt %)                                                                           0.91     0.91     0.91   0.91                                     Amount of sintering                                                                         P: 1.15  SiO.sub.2 : 1.41                                                                        B: 0.40                                                                               Ti: 0.55                                 preventive (wt %)                                                              Soluble Na salt                                                                             2256     2187     1458   1297                                     (ppm)                                                                          Soluble sulfate                                                                             2468     2678     2597   3018                                     (ppm)                                                                          pH value of  6.2      6.0      5.5    5.7                                      particles (-)                                                                  Resin adsorptivity                                                                          31.3     --       --     --                                       (%)                                                                            ______________________________________                                                                Comp.                                                                          Example                                                                        97                                                      ______________________________________                                         Kind of low-density    Comp.                                                   acicular hematite      Example                                                 particles              84                                                      Densification                                                                  Temperature (° C.)                                                                             450                                                     Time (min.)            30                                                      High-density acicular                                                          hematite particles                                                             Average major axial    0.135                                                   diameter (μm)                                                               Average minor axial    0.0189                                                  diameter (μm)                                                               Geometric standard     1.36                                                    deviation σg (-)                                                         Aspect ratio (-)       7.14                                                    S.sub.BET (m.sup.2 /g) 86.4                                                    S.sub.TEM (m.sup.2 /g) 43.5                                                    S.sub.BET /S.sub.TEM (-)                                                                              1.98                                                    Al content (wt %)      0.91                                                    Amount of sintering    P: 0.83                                                 preventive (wt %)                                                              Soluble Na salt        2156                                                    (ppm)                                                                          Soluble sulfate        2658                                                    (ppm)                                                                          pH value of            5.4                                                     particles (-)                                                                  Resin adsorptivity     --                                                      (%)                                                                            ______________________________________                                    

                  TABLE 27                                                         ______________________________________                                                    Exam-   Exam-   Exam-                                                          ple     ple     ple   Example                                                                               Example                                           143     144     145   146    147                                    ______________________________________                                         Kind of high-                                                                             Exam-   Exam-   Exam- Example                                                                               Example                                density acicular                                                                          ple     ple     ple   137    138                                    hematite particles                                                                        134     135     136                                                 Wet pulverization                                                              Yes or No  Yes     Yes     Yes   Yes    Yes                                    Amount of residue                                                                         0       0       0     0      0                                      on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.7    13.6    13.5  13.8   13.9                                   Temperature (° C.)                                                                 98      98      95    95     97                                     Time (min.)                                                                               180     240     180   210    180                                    ______________________________________                                                    Example   Example  Example Example                                             148       149      150     151                                      ______________________________________                                         Kind of high-                                                                             Example   Example  Example Example                                  density acicular                                                                          139       140      141     142                                      hematite particles                                                             Wet pulverization                                                              Yes or No  Yes       Yes      Yes     Yes                                      Amount of residue                                                                         0         0        0       0                                        on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              13.6      13.5     13.3    13.1                                     Temperature (° C.)                                                                 93        91       98      97                                       Time (min. )                                                                              150       180      240     180                                      ______________________________________                                    

                  TABLE 28                                                         ______________________________________                                                      Example  Example  Example                                                                               Example                                               143      144      145    146                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.134    0.120    0.121  0.141                                    diameter (μm)                                                               Average minor axial                                                                         0.0192   0.0193   0.0190 0.0203                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.36     1.36   1.44                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.98     6.22     6.37   6.95                                     S.sub.BET (m.sup.2 /g)                                                                      48.9     53.9     55.0   44.1                                     S.sub.TEM (m.sup.2 /g)                                                                      42.9     43.1     43.7   40.6                                     S.sub.BET /S.sub.TEM (-)                                                                    1.14     1.25     1.26   1.09                                     Al content (wt %)                                                                           0.91     2.51     2.53   1.35                                     Amount of sintering                                                                         P: 0.35  SiO.sub.2 : 1.16                                                                        SiO.sub.2 : 0.90                                                                      SiO.sub.2 : 1.66                         preventive (wt %)              P: 0.21                                         Soluble Na salt                                                                             89       72       68     118                                      (ppm)                                                                          Soluble sulfate                                                                             8        6        13     21                                       (ppm)                                                                          pH value of  9.3      9.4      9.3    9.7                                      particles (-)                                                                  Resin adsorptivity                                                                          83.8     86.8     89.1   90.3                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               147      148      149    150                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.141    0.110    0.108  0.162                                    diameter (μm)                                                               Average minor axial                                                                         0.0205   0.0176   0.0176 0.0240                                   diameter (μm)                                                               Geometric standard                                                                          1.45     1.31     1.32   1.42                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.88     6.25     6.14   6.75                                     S.sub.BET (m.sup.2 /g)                                                                      45.7     57.1     56.9   38.1                                     S.sub.TEM (m.sup.2 /g)                                                                      40.3     47.2     47.3   34.4                                     S.sub.BET /S.sub.TEM (-)                                                                    1.14     1.21     1.20   1.11                                     Al content (wt %)                                                                           1.31     4.98     4.93   1.01                                     Amount of sintering                                                                         Ti: 3.22 P: 0.32  B: 0.08                                                                               SiO.sub.2 : 1.25                         preventive (wt %)                                                              Soluble Na salt                                                                             90       134      116    43                                       (ppm)                                                                          Soluble sulfate                                                                             7        6        11     3                                        (ppm)                                                                          pH value of  9.3      9.5      9.0    9.0                                      particles (-)                                                                  Resin adsorptivity                                                                          81.4     92.1     93.8   83.9                                     (%)                                                                            ______________________________________                                                                Example                                                                        151                                                     ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial    0.164                                                   diameter (μm)                                                               Average minor axial    0.0240                                                  diameter (μm)                                                               Geometric standard     1.42                                                    deviation σg (-)                                                         Aspect ratio (-)       6.83                                                    S.sub.BET (m.sup.2 /g) 39.7                                                    S.sub.TEM (m.sup.2 /g) 34.4                                                    S.sub.BET /S.sub.TEM (-)                                                                              1.15                                                    Al content (wt %)      1.03                                                    Amount of sintering    SiO.sub.2 : 0.43                                        preventive (wt %)      P: 0.30                                                 Soluble Na salt        25                                                      (ppm)                                                                          Soluble sulfate        4                                                       (ppm)                                                                          pH value of            8.9                                                     particles (-)                                                                  Resin adsorptivity     86.9                                                    (%)                                                                            ______________________________________                                    

                  TABLE 29                                                         ______________________________________                                                    Comp.   Comp.   Comp.                                                          Exam-   Exam-   Exam- Comp.  Comp.                                             ple     ple     ple   Example                                                                               Example                                           98      99      100   101    102                                    ______________________________________                                         Kind of acicular                                                                          Comp.   Comp.   Comp. Comp.  Comp.                                  hematite particles                                                                        Exam-   Exam-   Exam- Example                                                                               Example                                           ple     ple     ple   92     94                                                89      90      91                                                  Wet pulverization                                                              Yes or No  Yes     Yes     Yes   No     No                                     Amount of residue                                                                         0       0       0     21.8   31.8                                   on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)                                                                              --      12.0    13.1  13.1   10.5                                   Temperature (° C.)                                                                 --      95      58    90     90                                     Time (min.)                                                                               --      180     180   180    180                                    ______________________________________                                                       Comp.     Comp.     Comp.                                                      Example   Example   Example                                                    103       104       105                                          ______________________________________                                         Kind of acicular                                                                             Comp.     Comp.     Comp.                                        hematite particles                                                                           Example   Example   Example                                                    95        96        97                                           Wet pulverization                                                              Yes or No     No        Yes       Yes                                          Amount of residue                                                                            29.7      0         0                                            on sieve (wt %)                                                                Heat treatment                                                                 with aqueous                                                                   alkali solution                                                                pH value (-)  13.3      13.3      9.1                                          Temperature (° C.)                                                                    90        40        95                                           Time (min.)   180       180       180                                          ______________________________________                                    

                  TABLE 30                                                         ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               98       99       100    101                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.214    0.211    0.212  0.213                                    diameter (μm)                                                               Average minor axial                                                                         0.0297   0.0298   0.0300 0.0300                                   diameter (μm)                                                               Geometric standard                                                                          1.41     1.42     1.43   1.42                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            7.21     7.08     7.07   7.10                                     S.sub.BET (m.sup.2 /g)                                                                      43.8     33.9     36.8   43.2                                     S.sub.TEM (m.sup.2 /g)                                                                      27.7     27.6     27.5   27.4                                     S.sub.BET /S.sub.TEM (-)                                                                    1.58     1.23     1.34   1.57                                     Al content (wt %)                                                                           0.004    0.004    0.004  0.004                                    Amount of sintering                                                                         P: 0.46  SiO.sub.2 : 1.24                                                                        SiO.sub.2 : 0.88                                                                      P: 0.40                                  preventive (wt %)                                                              Soluble Na salt                                                                             785      456      371    326                                      (ppm)                                                                          Soluble sulfate                                                                             489      325      173    168                                      (ppm)                                                                          pH value of  6.7      6.6      7.3    7.1                                      particles (-)                                                                  Resin adsorptivity                                                                          35.7     44.6     50.7   37.5                                     (%)                                                                            ______________________________________                                                      Comp.    Comp.    Comp.  Comp.                                                 Example  Example  Example                                                                               Example                                               102      103      104    105                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after heat                                                               treatment with                                                                 aqueous alkali                                                                 solution                                                                       Average major axial                                                                         0.134    0.135    0.131  0.135                                    diameter (μm)                                                               Average minor axial                                                                         0.0192   0.0188   0.0190 0.0189                                   diameter (μm)                                                               Geometric standard                                                                          1.37     1.36     1.35   1.36                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.98     7.18     6.89   7.14                                     S.sub.BET (m.sup.2 /g)                                                                      50.0     61.1     50.3   88.1                                     S.sub.TEM (m.sup.2 /g)                                                                      42.9     43.8     43.4   43.5                                     S.sub.BET /S.sub.TEM (-)                                                                    1.16     1.40     1.16   2.02                                     Al content (wt %)                                                                           0.91     0.91     0.91   0.91                                     Amount of sintering                                                                         SiO.sub.2 : 1.41                                                                        B: 0.18  Ti: 0.56                                                                              P: 0.84                                  preventive (wt %)                                                              Soluble Na salt                                                                             467      345      356    487                                      (ppm)                                                                          Soluble sulfate                                                                             256      177      170    290                                      (ppm)                                                                          pH value of  7.0      7.4      7.3    6.8                                      particles (-)                                                                  Resin adsorptivity                                                                          46.8     53.7     53.1   45.9                                     (%)                                                                            ______________________________________                                    

                  TABLE 31                                                         ______________________________________                                                      Example  Example  Example                                                                               Example                                               152      153      154    155                                      ______________________________________                                         Kind of acicular                                                                            Example  Example  Example                                                                               Example                                  hematite particles                                                                          143      144      145    146                                      treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         Sodium   #3 Water Sodium Colloidal                                             aluminate                                                                               glass    aluminate                                                                             silica                                   Amount added (wt %)                                                                         3.0      1.5      1.5    3.0                                      Coating material                                                               Kind         Al*      Si*      Al*    Si*                                      Amount (wt %)                                                                               2.91     1.45     1.47   2.89                                     ______________________________________                                                      Example 156      Example 157                                      ______________________________________                                         Kind of acicular                                                                            Example 147      Example 148                                      hematite particles                                                             treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         Aluminum acetate/                                                                               Sodium                                                        #3 Water glass   aluminate/                                                                     Colloidal silica                                 Amount added (wt %)                                                                         5.0/1.0          1.0/5.0                                          Coating material                                                               Kind         Al*/Si*          Al*/Si*                                          Amount (wt %)                                                                               4.73/0.96        0.98/4.66                                        ______________________________________                                                      Example                Example                                                 158      Example 159   160                                        ______________________________________                                         Kind of acicular                                                                            Example  Example 150   Example                                    hematite particles                                                                          149                    151                                        treated with aqueous                                                           alkali solution                                                                Surface treatment                                                              Kind         Sodium   Aluminum acetate/                                                                            Sodium                                                  aluminate                                                                               Colloidal silica                                                                             aluminate                                  Amount added (wt %)                                                                         10.0     1.5/2.0       0.5                                        Coating material                                                               Kind         Al*      Al*/Si*       Al*                                        Amount (wt %)                                                                               8.99     1.47/1.93     0.49                                       ______________________________________                                          Al*: aluminum hydroxide, Si*: silicon oxide                              

                  TABLE 32                                                         ______________________________________                                                      Example  Example  Example                                                                               Example                                               152      153      154    155                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                         0.134    0.121    0.121  0.141                                    diameter (μm)                                                               Average minor axial                                                                         0.0192   0.0193   0.0190 0.0202                                   diameter (μm)                                                               Geometric standard                                                                          1.35     1.36     1.36   1.44                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.98     6.27     6.37   6.98                                     S.sub.BET (m.sup.2 /g)                                                                      49.3     54.1     56.1   46.9                                     S.sub.TEM (m.sup.2 /g)                                                                      42.9     43.0     43.7   40.8                                     S.sub.BET /S.sub.TEM (-)                                                                    1.15     1.26     1.28   1.15                                     Al content (wt %)                                                                           0.91     2.51     2.53   1.35                                     Amount of sintering                                                                         P: 0.33  SiO.sub.2 : 1.16                                                                        SiO.sub.2 : 0.90                                                                      SiO.sub.2 : 1.66                         preventive (wt %)              P: 0.20                                         Soluble Na salt                                                                             56       63       78     96                                       (ppm)                                                                          Soluble sulfate                                                                             3        3        5      12                                       (ppm)                                                                          pH value of  9.2      9.4      9.3    9.1                                      particles (-)                                                                  Resin adsorptivity                                                                          85.1     87.0     90.1   92.5                                     (%)                                                                            ______________________________________                                                      Example  Example  Example                                                                               Example                                               156      157      158    159                                      ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                         0.140    0.109    0.108  0.162                                    diameter (μm)                                                               Average minor axial                                                                         0.0204   0.0176   0.0177 0.0239                                   diameter (μm)                                                               Geometric standard                                                                          1.44     1.31     1.31   1.42                                     deviation σg (-)                                                         Aspect ratio (-)                                                                            6.86     6.19     6.10   6.78                                     S.sub.BET (m.sup.2 /g)                                                                      46.0     58.3     59.7   39.5                                     S.sub.TEM (m.sup.2 /g)                                                                      40.5     47.2     47.0   34.6                                     S.sub.BET /S.sub.TEM (-)                                                                    1.14     1.23     1.27   1.14                                     Al content (wt %)                                                                           1.31     4.98     4.93   1.01                                     Amount of sintering                                                                         Ti: 3.04 P: 0.30  B: 0.07                                                                               SiO.sub.2 : 1.25                         preventive (wt %)                                                              Soluble Na salt                                                                             54       91       89     32                                       (ppm)                                                                          Soluble sulfate                                                                             4        5        2      8                                        (ppm)                                                                          pH value of  9.3      9.5      9.5    9.3                                      particles (-)                                                                  Resin adsorptivity                                                                          86.9     92.0     94.6   89.0                                     (%)                                                                            ______________________________________                                                           Example                                                                        160                                                          ______________________________________                                         Acicular hematite                                                              particles washed with                                                          water after surface                                                            treatment                                                                      Average major axial                                                                              0.163                                                        diameter (μm)                                                               Average minor axial                                                                              0.0240                                                       diameter (μm)                                                               Geometric standard                                                                               1.41                                                         deviation σg (-)                                                         Aspect ratio (-)  6.79                                                         S.sub.BET (m.sup.2 /g)                                                                           40.1                                                         S.sub.TEM (m.sup.2 /g)                                                                           34.4                                                         S.sub.BET /S.sub.TEM (-)                                                                         1.17                                                         Al content (wt %) 1.03                                                         Amount of sintering                                                                              SiO.sub.2 : 0.43                                             preventive (wt %) P: 0.30                                                      Soluble Na salt   27                                                           (ppm)                                                                          Soluble sulfate   4                                                            (ppm)                                                                          pH value of       9.1                                                          particles (-)                                                                  Resin adsorptivity                                                                               90.4                                                         (%)                                                                            ______________________________________                                    

                  TABLE 33                                                         ______________________________________                                                 Example  Example  Example                                                                               Example                                                                               Example                                        161      162      163    164    165                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Example  Example  Example                                                                               Example                                                                               Example                                acicular                                                                               143      144      145    146    147                                    hematite                                                                       particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              384      435      410    307    333                                    (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.4      3.3      3.4    3.4    3.2                                    (μm)                                                                        Gloss (%)                                                                              207      211      215    207    209                                    Ra (nm) 6.4      6.0      6.0    6.4    6.6                                    Young's 128      124      125    129    131                                    modulus (-)                                                                    (relative                                                                      value)                                                                         ______________________________________                                                 Example  Example  Example                                                                               Example                                                                               Example                                        166      167      168    169    170                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Example  Example  Example                                                                               Example                                                                               Example                                acicular                                                                               148      149      150    151    152                                    hematite                                                                       particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              512      563      256    256    282                                    (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.5      3.3      3.3    3.4    3.3                                    (μm)                                                                        Gloss (%)                                                                              218      216      202    205    212                                    Ra (nm) 5.8      5.7      6.1    6.2    5.9                                    Young's 122      121      131    132    129                                    modulus (-)                                                                    (relative                                                                      value)                                                                         ______________________________________                                                 Example  Example  Example                                                                               Example                                                                               Example                                        171      172      173    174    175                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Example  Example  Example                                                                               Example                                                                               Example                                acicular                                                                               153      154      155    156    157                                    hematite                                                                       particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              333      307      205    230    486                                    (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.2      3.3      3.2    3.2    3.4                                    (μm)                                                                        Gloss (%)                                                                              218      223      217    218    225                                    Ra (nm) 5.6      5.7      6.0    6.2    5.5                                    Young's 125      125      130    131    125                                    modulus (-)                                                                    (relative                                                                      value)                                                                         ______________________________________                                                      Example  Example       Example                                                 176      177           178                                        ______________________________________                                         Production of                                                                  non-magnetic                                                                   coating film                                                                   composition                                                                    Kind of acicular                                                                            Example  Example       Example                                    hematite     158      159           160                                        particles                                                                      Weight ratio of                                                                             5.0      5.0           5.0                                        particles and                                                                  resin (-)                                                                      Non-magnetic                                                                   coating film                                                                   composition                                                                    Viscosity (cP)                                                                              435      230           205                                        Non-magnetic                                                                   undercoat layer                                                                Thickness (μm)                                                                           3.3      3.3           3.4                                        Gloss (%)    227      209           210                                        Ra (nm)      5.4      6.0           5.8                                        Young's modulus                                                                             124      133           133                                        (-)                                                                            (relative value)                                                               ______________________________________                                    

                  TABLE 34                                                         ______________________________________                                                 Comp.    Comp.    Comp.  Comp.  Comp.                                          Example  Example  Example                                                                               Example                                                                               Example                                        106      107      108    109    110                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Starting Comp.    Comp.  Comp.  Comp.                                  acicular                                                                               material Example  Example                                                                               Example                                                                               Example                                hematite                                                                               (V)      85       86     73     87                                     particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              10240    128      154    14080  207                                    (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.7      3.2      3.3    3.6    3.3                                    (μm)                                                                        Gloss (%)                                                                              35       42       68     73     135                                    Ra (nm) 118.0    95.4     65.2   47.2   37.8                                   Young's 96       98       103    100    113                                    modulus                                                                        (-)                                                                            (relative                                                                      value)                                                                         ______________________________________                                                 Comp.    Comp.    Comp.  Comp.  Comp.                                          Example  Example  Example                                                                               Example                                                                               Example                                        111      112      113    114    115                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Comp.    Comp.    Comp.  Comp.  Comp.                                  acicular                                                                               Example  Example  Example                                                                               Example                                                                               Example                                hematite                                                                               88       98       99     100    101                                    particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              384      973      333    384    435                                    (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.4      3.3      3.2    3.3    3.3                                    (μm)                                                                        Gloss (%)                                                                              146      172      175    171    154                                    Ra (nm) 29.0     17.0     13.0   14.2   32.6                                   Young's 110      115      119    113    108                                    modulus                                                                        (-)                                                                            (relative                                                                      value)                                                                         ______________________________________                                                 Comp.    Comp.    Comp.  Comp.  Comp.                                          Example  Example  Example                                                                               Example                                                                               Example                                        116      117      118    119    120                                    ______________________________________                                         Production                                                                     of non-                                                                        magnetic                                                                       coating film                                                                   composition                                                                    Kind of Comp.    Comp.    Comp.  Comp.  Comp.                                  acicular                                                                               Example  Example  Example                                                                               Example                                                                               Example                                hematite                                                                               93       102      103    104    105                                    particles                                                                      Weight ratio                                                                           5.0      5.0      5.0    5.0    5.0                                    of particles                                                                   and resin (-)                                                                  Non-                                                                           magnetic                                                                       coating film                                                                   composition                                                                    Viscosity                                                                              512      410      768    384    1229                                   (cP)                                                                           Non-                                                                           magnetic                                                                       undercoat                                                                      layer                                                                          Thickness                                                                              3.4      3.3      3.4    3.3    3.5                                    (μm)                                                                        Gloss (%)                                                                              164      185      188    194    191                                    Ra (nm) 19.6     10.8     10.2   9.2    9.6                                    Young's 96       116      113    118    108                                    modulus                                                                        (-)                                                                            (relative                                                                      value)                                                                         ______________________________________                                    

                  TABLE 35                                                         ______________________________________                                                  Kind of magnetic particles                                                     magnetic                                                                               magnetic magnetic  magnetic                                            iron-based                                                                             iron-based                                                                              iron-based                                                                               iron-based                                          alloy   alloy    alloy     alloy                                               particles                                                                              particles                                                                               particles particles                                           (a)     (b)      (c)       (d)                                        ______________________________________                                         Average major                                                                             0.110     0.098    0.101   0.125                                    axial diameter                                                                 (μm)                                                                        Average minor                                                                             0.0150    0.0134   0.0144  0.0184                                   axial diameter                                                                 (μm)                                                                        Aspect ratio (-)                                                                          7.33      7.31     7.01    6.79                                     Geometric  1.36      1.35     1.38    1.35                                     standard                                                                       deviation (-)                                                                  Coercive force                                                                            1915      1938     2065    1896                                     (Oe)                                                                           Saturation 131.6     130.5    128.9   130.8                                    magnetization                                                                  (emu/g)                                                                        Content of                                                                     existent Al                                                                    Central portion                                                                           2.61      1.32     1.38    0.01                                     (wt %)                                                                         Surface layer                                                                             1.36      2.84     2.65    0.01                                     portion (wt %)                                                                 Surface coating                                                                           0.01      0.01     0.78    0.01                                     film (wt %)                                                                    Content of 0.01      0.36     2.78    0.01                                     existent Nd (wt %)                                                             Resin      72.5      80.1     83.6    57.6                                     adsorptivity (%)                                                               ______________________________________                                    

                  TABLE 36                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                179     180     181   182   183                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex.     Ex.     Ex.   Ex.   Ex.                                   undercoat layer                                                                             161     162     163   164   165                                   Kind of magnetic iron-                                                                      *       (a)     (a)   (a)   (a)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.0     1.1     1.1   1.2   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1978    1990    1985  1978  1983                                  Br/Bm (-)    0.88    0.88    0.88  0.88  0.89                                  Gloss (%)    232     236     236   228   230                                   Ra (nm)      6.2     6.0     6.0   6.4   6.4                                   Young's modulus (-)                                                                         132     128     130   133   134                                   (relative value)                                                               Durability                                                                     Running durability                                                                          30≦                                                                             29.6    25.8  28.4  26.5                                  (min)                                                                          Scratch resistance                                                                          A       A       B     A     B                                     Linear absorption                                                                           1.23    1.24    1.25  1.28  1.30                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     3.6     4.6     5.3   5.1   6.2                                   coercive force (%)                                                             Percentage of change in                                                                     4.8     6.9     5.8   5.9   7.5                                   Bm (%)                                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                184     185     186   187   188                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex.     Ex.     Ex.   Ex.   Ex.                                   undercoat layer                                                                             166     167     168   169   170                                   Kind of magnetic iron-                                                                      *       *       (b)   (b)   (a)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.1     1.1   1.0   1.0                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1967    1971    1998  1989  1965                                  Br/Bm (-)    0.88    0.88    0.89  0.89  0.88                                  Gloss (%)    231     235     219   223   238                                   Ra (nm)      6.2     6.0     5.8   6.0   5.8                                   Young's modulus (-)                                                                         127     125     134   133   134                                   (relative value)                                                               Durability                                                                     Running durability                                                                          30≦                                                                             30≦                                                                             30≦                                                                           30≦                                                                           29.4                                  (min)                                                                          Scratch resistance                                                                          A       A       A     A     A                                     Linear absorption                                                                           1.22    1.23    1.31  1.33  1.24                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     4.8     4.9     3.7   2.1   6.4                                   coercive force (%)                                                             Percentage of change in                                                                     5.1     3.9     5.5   4.1   5.6                                   Bm (%)                                                                         ______________________________________                                                      Ex.     Ex.     Ex.   Ex.   Ex.                                                189     190     191   192   193                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Ex.     Ex.     Ex.   Ex.   Ex.                                   undercoat layer                                                                             171     172     173   174   175                                   Kind of magnetic iron-                                                                      (b)     (b)     (c)   (c)   (c)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.0     1.1     1.1   1.1   1.1                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1987    1985    2135  2148  2153                                  Br/Bm (-)    0.88    0.89    0.89  0.89  0.90                                  Gloss (%)    240     242     235   232   245                                   Ra (nm)      5.4     5.2     5.9   5.9   5.5                                   Young's modulus (-)                                                                         130     128     134   135   129                                   (relative value)                                                               Durability                                                                     Running durability                                                                          30≦                                                                             30≦                                                                             30≦                                                                           30≦                                                                           30≦                            (min)                                                                          Scratch resistance                                                                          A       A       A     A     A                                     Linear absorption                                                                           1.25    1.24    1.28  1.30  1.23                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     4.1     5.3     2.6   1.8   2.5                                   coercive force (%)                                                             Percentage of change in                                                                     5.7     4.3     4.0   3.2   2.7                                   Bm (%)                                                                         ______________________________________                                                         Ex.     Ex.          Ex.                                                       194     195          196                                       ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                           Ex.     Ex.          Ex.                                       undercoat layer 176     177          178                                       Kind of magnetic iron-                                                                         (c)     (c)          (c)                                       based metal particles                                                          Weight ratio of 5.0     5.0          5.0                                       magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                          1.1     1.0          1.0                                       layer (μm)                                                                  Coercive force (Oe)                                                                            2121    2118         2147                                      Br/Bm (-)       0.91    0.90         0.89                                      Gloss (%)       241     228          226                                       Ra (nm)         5.9     6.0          6.2                                       Young's modulus (-)                                                                            127     139          138                                       (relative value)                                                               Durability                                                                     Running durability                                                                             30≦                                                                             30≦   30≦                                (min)                                                                          Scratch resistance                                                                             A       A            A                                         Linear absorption                                                                              1.22    1.34         1.33                                      coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                        2.7     1.6          2.7                                       coercive force (%)                                                             Percentage of change in                                                                        2.5     1.8          1.4                                       Bm (%)                                                                         ______________________________________                                          (Note)                                                                         *: Particles in Example 124                                                    (a): magnetic ironbased metal particles (a)                                    (b): magnetic ironbased metal particles (b)                                    (c): magnetic ironbased metal particles (c)                              

                  TABLE 37                                                         ______________________________________                                                      Comp.   Comp.   Comp. Comp. Comp.                                              Ex.     Ex.     Ex.   Ex.   Ex.                                                121     122     123   124   125                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Comp.   Comp.   Comp. Comp. Comp.                                 undercoat layer                                                                             Ex.     Ex.     Ex.   Ex.   Ex.                                                106     107     108   109   110                                   Kind of magnetic iron-                                                                      *       (a)     (a)   (a)   (a)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.3     1.2     1.2   1.2   1.2                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1910    1923    1934  1932  1940                                  Br/Bm (-)    0.79    0.78    0.79  0.80  0.83                                  Gloss (%)    134     142     164   168   177                                   Ra (nm)      73.0    69.7    41.6  38.9  26.4                                  Young's modulus (-)                                                                         102     102     108   115   119                                   (relative value)                                                               Durability                                                                     Running durability                                                                          0.6     0.6     0.8   3.7   6.0                                   (min)                                                                          Scratch resistance                                                                          D       D       D     D     D                                     Linear absorption                                                                           0.82    0.97    1.02  1.04  1.06                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     47.5    36.7    38.0  32.6  23.1                                  coercive force (%)                                                             Percentage of change in                                                                     35.8    45.1    38.5  34.6  31.4                                  Bm (%)                                                                         ______________________________________                                                      Comp.   Comp.   Comp. Comp. Comp.                                              Ex.     Ex.     Ex.   Ex.   Ex.                                                126     127     128   129   130                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Comp.   Comp.   Comp. Comp. Comp.                                 undercoat layer                                                                             Ex.     Ex.     Ex.   Ex.   Ex.                                                111     112     113   114   115                                   Kind of magnetic iron-                                                                      (a)     (a)     (a)   (a)   (a)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.3     1.1     1.3   1.1   1.2                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1947    1934    1965  1972  1943                                  Br/Bm (-)    0.84    0.85    0.85  0.85  0.83                                  Gloss (%)    179     193     197   192   169                                   Ra (nm)      25.0    16.3    11.2  12.0  28.6                                  Young's modulus (-)                                                                         113     119     124   116   113                                   (relative value)                                                               Durability                                                                     Running durability                                                                          4.6     10.2    13.5  9.0   9.3                                   (min)                                                                          Scratch resistance                                                                          D       C       C     D     C                                     Linear absorption                                                                           1.08    1.17    1.18  1.16  1.15                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     34.2    20.1    14.6  17.4  18.8                                  coercive force (%)                                                             Percentage of change in                                                                     26.8    28.2    18.7  18.7  21.5                                  Bm (%)                                                                         ______________________________________                                                      Comp.   Comp.   Comp. Comp. Comp.                                              Ex.     Ex.     Ex.   Ex.   Ex.                                                131     132     133   134   135                                   ______________________________________                                         Magnetic recording                                                             medium using magnetic                                                          iron-based metal particles                                                     Kind of non-magnetic                                                                        Comp.   Comp.   Comp. Comp. Comp.                                 undercoat layer                                                                             Ex.     Ex.     Ex.   Ex.   Ex.                                                116     117     118   119   120                                   Kind of magnetic iron-                                                                      (d)     (d)     (d)   (d)   (d)                                   based metal particles                                                          Weight ratio of                                                                             5.0     5.0     5.0   5.0   5.0                                   magnetic particles and                                                         resin (-)                                                                      Thickness of magnetic                                                                       1.1     1.0     1.3   1.3   1.2                                   layer (μm)                                                                  Coercive force (Oe)                                                                         1896    1906    1893  1899  1913                                  Br/Bm (-)    0.85    0.85    0.86  0.86  0.85                                  Gloss (%)    180     196     206   213   215                                   Ra (nm)      21.7    10.8    9.6   8.8   8.2                                   Young's modulus (-)                                                                         99      121     119   123   116                                   (relative value)                                                               Durability                                                                     Running durability                                                                          13.5    16.5    18.3  19.6  17.4                                  (min)                                                                          Scratch resistance                                                                          C       C       C     C     C                                     Linear absorption                                                                           1.11    1.21    1.23  1.23  1.24                                  coefficient (μm.sup.-1)                                                     Corrosiveness                                                                  Percentage of change in                                                                     37.9    21.8    18.6  14.8  18.0                                  coercive force (%)                                                             Percentage of change in                                                                     32.8    28.0    27.9  21.8  16.9                                  Bm (%)                                                                         ______________________________________                                          (Note)                                                                         *: Particles in Examples 124                                                   (a): magnetic ironbased metal particles (a)                                    (d): magnetic ironbased metal particles (d)                               

What is claimed is:
 1. Acicular hematite particles containing 0.05 to 50% by weight of aluminum, calculated as Al, approximately uniformly within the particle; having an average major axial diameter of not more than 0.3 μm and a pH value of the particles of not less than 8, and containing not more than 300 ppm soluble sodium salts, calculated as Na, and not more than 150 ppm soluble sulfates, calculated as SO₄.
 2. Acicular hematite particles according to claim 1, which further have an average minor axial diameter of 0.0025 to 0.15 μm, an aspect ratio of not less than 2:1, a BET specific surface area (S_(BET)) of not less than 35 m² /g, a geometrical standard deviation of not more than 1.50, a resin adsorptivity of not less than 65%, and an S_(BET) /S_(TEM) value of 0.5 to 2.5, wherein said S_(TEM) is a value calculated from the major axial diameter and the minor axial diameter of said particles which were measured from said particles in an electron micrograph.
 3. Acicular hematite particles according to claim 1, which contain aluminum of 0.5 to 50% by weight, calculated as Al within the particles, have an average major axial diameter of 0.005 to 0.3 μm and a pH value of the particles of 8.5 to 12, and contain soluble sodium salts of 0.01 to 250 ppm soluble sodium, calculated as Na and soluble sulfates of 0.01 to 70 ppm soluble sulfate, calculated as SO₄.
 4. Acicular hematite particles according to claim 1, which further have an average minor axial diameter of 0.01 to 0.10 μm, an aspect ratio of 3:1 to 20:1, a BET specific surface area of 40 to 150 m² /g, a geometrical standard deviation of 1.01 to 1.40, a resin adsorptivity of not less than 70%, and an S_(BET) /S_(TEM) value of 0.7 to 2.0.
 5. Acicular hematite particles according to claim 1, which further have a coating comprising at least one selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon, which is coated on the surfaces of said acicular hematite particles.
 6. Acicular hematite particles according to claim 5, wherein the amount of coating material is 0.01 to 50 wt % calculated as either of Al and SiO₂ based on the total weight of particles. 